## 1. Hume's Problem



Hume introduces the problem of induction as part of an analysis of the
notions of cause and effect. Hume worked with a picture, widespread in
the early modern period, in which the mind was populated with mental
entities called "ideas". Hume thought that ultimately all
our ideas could be traced back to the "impressions" of
sense experience. In the simplest case, an idea enters the mind by
being "copied" from the corresponding impression (T.
1.1.1.7/4). More complex ideas are then created by the combination of
simple ideas (E. 2.5/19). Hume took there to be a number of relations
between ideas, including the relation of causation (E. 3.2). (For more
on Hume's philosophy in general, see Morris & Brown
2014).



For Hume, the relation of causation is the only relation by means of
which "we can go beyond the evidence of our memory and
senses" (E. 4.1.4, T. 1.3.2.3/74). Suppose we have an object
present to our senses: say gunpowder. We may then infer to an effect
of that object: say, the explosion. The causal relation links our past
and present experience to our expectations about the future (E.
4.1.4/26).



Hume argues that we cannot make a causal inference by purely *a
priori* means (E. 4.1.7). Rather, he claims, it is based on
experience, and specifically experience of constant conjunction. We
infer that the gunpowder will explode on the basis of past experience
of an association between gunpowder and explosions.



Hume wants to know more about the basis for this kind of inference. If
such an inference is made by a "chain of reasoning" (E.
4.2.16), he says, he would like to know what that reasoning is. In
general, he claims that the inferences depend on a transition of the
form:



> 
> 
> *I have found that such an object has always been attended with
> such an effect, and I foresee, that other objects, which are, in
> appearance, similar, will be attended with similar effects*. (E.
> 4.2.16)
> 
> 
> 



In the *Treatise*, Hume says that



> 
> 
> if Reason determin'd us, it would proceed upon that principle
> *that instances, of which we have had no experience, must resemble
> those, of which we have had experience, and that the course of nature
> continues always uniformly the same*. (T. 1.3.6.4)
> 
> 
> 



For convenience, we will refer to this claim of similarity or
resemblance between observed and unobserved regularities as the
"Uniformity Principle (UP)". Sometimes it is also called
the "Resemblance Principle", or the "Principle of
Uniformity of Nature".



Hume then presents his famous argument to the conclusion that there
can be no reasoning behind this principle. The argument takes the form
of a dilemma. Hume makes a distinction between relations of ideas and
matters of fact. Relations of ideas include geometric, algebraic and
arithmetic propositions, "and, in short, every affirmation,
which is either intuitively or demonstratively certain".
"Matters of fact", on the other hand are empirical
propositions which can readily be conceived to be other than they are.
Hume says that



> 
> 
> All reasonings may be divided into two kinds, namely, demonstrative
> reasoning, or that concerning relations of ideas, and moral reasoning,
> or that concerning matter of fact and existence. (E. 4.2.18)
> 
> 
> 



Hume considers the possibility of each of these types of reasoning in
turn, and in each case argues that it is impossible for it to supply
an argument for the Uniformity Principle.



First, Hume argues that the reasoning cannot be demonstrative, because
demonstrative reasoning only establishes conclusions which cannot be
conceived to be false. And, he says,



> 
> 
> it implies no contradiction that the course of nature may change, and
> that an object seemingly like those which we have experienced, may be
> attended with different or contrary effects. (E. 4.2.18)
> 
> 
> 



It is possible, he says, to clearly and distinctly conceive of a
situation where the unobserved case does not follow the regularity so
far observed (E. 4.2.18, T. 1.3.6.5/89).



Second, Hume argues that the reasoning also cannot be "such as
regard matter of fact and real existence". He also calls this
"probable" reasoning. All such reasoning, he claims,
"proceed upon the supposition, that the future will be
conformable to the past", in other words on the Uniformity
Principle (E. 4.2.19).



Therefore, if the chain of reasoning is based on an argument of this
kind it will again be relying on this supposition, "and taking
that for granted, which is the very point in question". (E.
4.2.19, see also T. 1.3.6.7/90). The second type of reasoning then
fails to provide a chain of reasoning which is not circular.



In the Treatise version, Hume concludes



> 
> 
> Thus, not only our reason fails us in the discovery of the
> *ultimate connexion* of causes and effects, but even after
> experience has inform'd us of their *constant
> conjunction*, 'tis impossible for us to satisfy ourselves by
> our reason, why we shou'd extend that experience beyond those
> particular instances, which have fallen under our observation. (T.
> 1.3.6.11/91-2)
> 
> 
> 



The conclusion then is that our tendency to project past regularities
into the future is not underpinned by reason. The problem of induction
is to find a way to avoid this conclusion, despite Hume's
argument.



After presenting the problem, Hume does present his own
"solution" to the doubts he has raised (E. 5, T.
1.3.7-16). This consists of an explanation of what the inductive
inferences are driven by, if not reason. In the *Treatise* Hume
raises the problem of induction in an explicitly contrastive way. He
asks whether the transition involved in the inference is produced



> 
> 
> by means of the understanding or imagination; whether we are
> determin'd by reason to make the transition, or by a certain
> association and relation of perceptions? (T. 1.3.6.4)
> 
> 
> 



And he goes on to summarize the conclusion by saying



> 
> 
> When the mind, therefore, passes from the idea or impression of one
> object to the idea or belief of another, it is not determin'd by
> reason, but by certain principles, which associate together the ideas
> of these objects, and unite them in the imagination. (T. 1.3.6.12)
> 
> 
> 



Thus, it is the imagination which is taken to be responsible for
underpinning the inductive inference, rather than reason.



In the *Enquiry*, Hume suggests that the step taken by the
mind,



> 
> 
> which is not supported by any argument, or process of the
> understanding ... must be induced by some other principle of
> equal weight and authority. (E. 5.1.2)
> 
> 
> 



That principle is "custom" or "habit". The
idea is that if one has seen similar objects or events constantly
conjoined, then the mind is inclined to expect a similar regularity to
hold in the future. The tendency or "propensity" to draw
such inferences, is the effect of custom:



> 
> 
> ... having found, in many instances, that any two kinds of
> objects, flame and heat, snow and cold, have always been conjoined
> together; if flame or snow be presented anew to the senses, the mind
> is carried by custom to expect heat or cold, and to *believe*,
> that such a quality does exist and will discover itself upon a nearer
> approach. This belief is the necessary result of of placing the mind
> in such circumstances. It is an operation of the soul, when we are so
> situated, as unavoidable as to feel the passion of love, when we
> receive benefits; or hatred, when we meet with injuries. All these
> operations are a species of natural instincts, which no reasoning or
> process of the thought and understanding is able, either to produce,
> or to prevent. (E. 5.1.8)
> 
> 
> 



Hume argues that the fact that these inferences do follow the course
of nature is a kind of "pre-established harmony" (E.
5.2.21). It is a kind of natural instinct, which may in fact be more
effective in making us successful in the world, than if we relied on
reason to make these inferences.


## 2. Reconstruction



Hume's argument has been presented and formulated in many
different versions. There is also an ongoing lively discussion over
the historical interpretation of what Hume himself intended by the
argument. It is therefore difficult to provide an unequivocal and
uncontroversial reconstruction of Hume's argument. Nonetheless,
for the purposes of organizing the different responses to Hume's
problem that will be discussed in this article, the following
reconstruction will serve as a useful starting point.



Hume's argument concerns specific inductive inferences such
as:




All observed instances of *A* have been *B*.



The next instance of *A* will be *B*.




Let us call this "inference *I*". Inferences which
fall under this type of schema are now often referred to as cases of
"simple enumerative induction".



Hume's own example is:




All observed instances of bread (of a particular appearance) have been
nourishing.



The next instance of bread (of that appearance) will be
nourishing.




Hume's argument then proceeds as follows (premises are labeled
as P, and subconclusions and conclusions as C):




P1.
There are only two kinds of arguments: demonstrative and probable
(Hume's fork).
P2.
Inference *I* presupposes the Uniformity Principle
(UP).

*1st horn:*



P3. A
demonstrative argument establishes a conclusion whose negation is a
contradiction.
P4. The
negation of the UP is not a contradiction.
C1.There is no
demonstrative argument for the UP (by P3 and P4).


*2nd horn:*



P5. Any
probable argument for UP presupposes UP.
P6. An argument
for a principle may not presuppose the same principle
(Non-circularity).
C2. There is
no probable argument for the UP (by P5 and P6).


*Consequences:*



C3. There is no argument
for the UP (by P1, C1 and C2).
P7. If there is no
argument for the UP, there is no chain of reasoning from the premises
to the conclusion of any inference that presupposes the UP.
C4. There is
no chain of reasoning from the premises to the conclusion of inference
*I* (by P2, C3 and P7).
P8. If there
is no chain of reasoning from the premises to the conclusion of
inference *I*, the inference is not justified.
C5. Inference
*I* is not justified (by C4 and P8).



There have been different interpretations of what Hume means by
"demonstrative" and "probable" arguments.
Sometimes "demonstrative" is equated with
"deductive", and probable with "inductive"
(e.g., Salmon 1966). Then the first horn of Hume's dilemma would
eliminate the possibility of a deductive argument, and the second
would eliminate the possibility of an inductive argument. However,
under this interpretation,
 premise P3
 would not hold, because it is possible for the conclusion of a
deductive argument to be a non-necessary proposition. Premise
 P3
 could be modified to say that a demonstrative (deductive) argument
establishes a conclusion that cannot be false if the premises are
true. But then it becomes possible that the supposition that the
future resembles the past, which is not a necessary proposition, could
be established by a deductive argument from some premises, though not
from *a priori* premises (in contradiction to conclusion
 C1).



Another common reading is to equate "demonstrative" with
"deductively valid with *a priori* premises", and
"probable" with "having an empirical premise"
(e.g., Okasha 2001). This may be closer to the mark, if one thinks, as
Hume seems to have done, that premises which can be known *a
priori* cannot be false, and hence are necessary. If the inference
is deductively valid, then the conclusion of the inference from *a
priori* premises must also be necessary. What the first horn of
the dilemma then rules out is the possibility of a deductively valid
argument with *a priori* premises, and the second horn rules
out any argument (deductive or non-deductive), which relies on an
empirical premise.



However, recent commentators have argued that in the historical
context that Hume was situated in, the distinction he draws between
demonstrative and probable arguments has little to do with whether or
not the argument has a deductive form (Owen 1999; Garrett 2002). In
addition, the class of inferences that establish conclusions whose
negation is a contradiction may include not just deductively valid
inferences from *a priori* premises, but any inferences that
can be drawn using *a priori* reasoning (that is, reasoning
where the transition from premises to the conclusion makes no appeal
to what we learn from observations). It looks as though Hume does
intend the argument of the first horn to rule out any *a
priori* reasoning, since he says that a change in the course of
nature cannot be ruled out "by any demonstrative argument or
abstract reasoning *a priori*" (E. 5.2.18). On this
understanding, *a priori* arguments would be ruled out by the
first horn of Hume's dilemma, and empirical arguments by the
second horn. This is the interpretation that I will adopt for the
purposes of this article.



In Hume's argument, the UP plays a central role. As we will see
in
 section 4.2,
 various authors have been doubtful about this principle. Versions of
Hume's argument have also been formulated which do not make
reference to the UP. Rather they directly address the question of what
arguments can be given in support of the transition from the premises
to the conclusion of the specific inductive inference *I*. What
arguments could lead us, for example, to infer that the next piece of
bread will nourish from the observations of nourishing bread made so
far? For the first horn of the argument, Hume's argument can be
directly applied. A demonstrative argument establishes a conclusion
whose negation is a contradiction. The negation of the conclusion of
the inductive inference is not a contradiction. It is not a
contradiction that the next piece of bread is not nourishing.
Therefore, there is no demonstrative argument for the conclusion of
the inductive inference. In the second horn of the argument, the
problem Hume raises is a circularity. Even if Hume is wrong that all
inductive inferences depend on the UP, there may still be a
circularity problem, but as we shall see in
 section 4.1,
 the exact nature of the circularity needs to be carefully considered.
But the main point at present is that the Humean argument is often
formulated without invoking the UP.



Since Hume's argument is a dilemma, there are two main ways to
resist it. The first is to tackle the first horn and to argue that
there is after all a demonstrative argument -here taken to mean
an argument based on *a priori* reasoning--that can
justify the inductive inference. The second is to tackle the second
horn and to argue that there is after all a probable (or empirical)
argument that can justify the inductive inference. We discuss the
different variants of these two approaches in sections
 3
 and
 4.



There are also those who dispute the consequences of the dilemma. For
example, some scholars have denied that Hume should be read as
invoking a premise such
 premise P8
 at all. The reason, they claim, is that he was not aiming for an
explicitly normative conclusion about justification such as
 C5.
 Hume certainly is seeking a "chain of reasoning" from the
premises of the inductive inference to the conclusion, and he thinks
that an argument for the UP is necessary to complete the chain.
However, one could think that there is no further premise regarding
justification, and so the conclusion of his argument is simply
 C4:
 there is no chain of reasoning from the premises to the conclusion of
an inductive inference. Hume could then be, as Don Garrett and David
Owen have argued, advancing a "thesis in cognitive
psychology", rather than making a normative claim about
justification (Owen 1999; Garrett 2002). The thesis is about the
nature of the cognitive process underlying the inference. According to
Garrett, the main upshot of Hume's argument is that there can be
no reasoning process that establishes the UP. For Owen, the message is
that the inference is not drawn through a chain of ideas connected by
mediating links, as would be characteristic of the faculty of
reason.



There are also interpreters who have argued that Hume is merely trying
to exclude a specific kind of justification of induction, based on a
conception of reason predominant among rationalists of his time,
rather than a justification in general (Beauchamp & Rosenberg
1981; Baier 2009). In particular, it has been claimed that it is
"an attempt to refute the rationalist belief that at least some
inductive arguments are demonstrative" (Beauchamp &
Rosenberg 1981: xviii). Under this interpretation,
 premise P8
 should be modified to read something like:


* If there is no chain of reasoning based on demonstrative arguments
from the premises to the conclusion of inference *I*, then
inference *I* is not justified.



Such interpretations do however struggle with the fact that
Hume's argument is explicitly a two-pronged attack, which
concerns not just demonstrative arguments, but also probable
arguments.



The question of how expansive a normative conclusion to attribute to
Hume is a complex one. It depends in part on the interpretation of
Hume's own solution to his problem. As we saw in
 section 1,
 Hume attributes the basis of inductive inference to principles of the
imagination in the Treatise, and in the Enquiry to
"custom", "habit", conceived as a kind of
natural instinct. The question is then whether this alternative
provides any kind of justification for the inference, even if not one
based on reason. On the face of it, it looks as though Hume is
suggesting that inductive inferences proceed on an entirely arational
basis. He clearly does not think that they do not succeed in producing
good outcomes. In fact, Hume even suggests that this operation of the
mind may even be less "liable to error and mistake" than
if it were entrusted to "the fallacious deductions of our
reason, which is slow in its operations" (E. 5.2.22). It is also
not clear that he sees the workings of the imagination as completely
devoid of rationality. For one thing, Hume talks about the imagination
as governed by *principles*. Later in the *Treatise*, he
even gives "rules" and "logic" for
characterizing what should count as a good causal inference (T.
1.3.15). He also clearly sees it as possible to distinguish between
better forms of such "reasoning", as he continues to call
it. Thus, there may be grounds to argue that Hume was not trying to
argue that inductive inferences have no rational foundation
whatsoever, but merely that they do not have the specific type of
rational foundation which is rooted in the faculty of Reason.



All this indicates that there is room for debate over the intended
scope of Hume's own conclusion. And thus there is also room for
debate over exactly what form a premise (such as
 premise P8)
 that connects the rest of his argument to a normative conclusion
should take. No matter who is right about this however, the fact
remains that Hume has throughout history been predominantly read as
presenting an argument for inductive skepticism.



There are a number of approaches which effectively, if not explicitly,
take issue with
 premise P8
 and argue that providing a chain of reasoning from the premises to
the conclusion is not a necessary condition for justification of an
inductive inference. According to this type of approach, one may admit
that Hume has shown that inductive inferences are not justified in the
sense that we have reasons to think their conclusions true, but still
think that weaker kinds of justification of induction are possible
 (section 5).
 Finally, there are some philosophers who do accept the skeptical
conclusion
 C5
 and attempt to accommodate it. For example, there have been attempts
to argue that inductive inference is not as central to scientific
inquiry as is often thought
 (section 6).


## 3. Tackling the First Horn of Hume's Dilemma



The first horn of Hume's argument, as formulated above, is aimed
at establishing that there is no demonstrative argument for the UP.
There are several ways people have attempted to show that the first
horn does not definitively preclude a demonstrative or *a
priori* argument for inductive inferences. One possible escape
route from the first horn is to deny
 premise P3,
 which amounts to admitting the possibility of synthetic *a
priori* propositions
 (section 3.1).
 Another possibility is to attempt to provide an *a priori*
argument that the conclusion of the inference is probable, though not
certain. The first horn of Hume's dilemma implies that there
cannot be a demonstrative argument to the conclusion of an inductive
inference because it is possible to conceive of the negation of the
conclusion. For instance, it is quite possible to imagine that the
next piece of bread I eat will poison me rather than nourish me.
However, this does not rule out the possibility of a demonstrative
argument that establishes only that the bread is highly likely to
nourish, not that it definitely will. One might then also challenge
 premise P8,
 by saying that it is not necessary for justification of an inductive
inference to have a chain of reasoning from its premises to its
conclusion. Rather it would suffice if we had an argument from the
premises to the claim that the conclusion is probable or likely. Then
an *a priori* justification of the inductive inference would
have been provided. There have been attempts to provide *a
priori* justifications for inductive inference based on Inference
to the Best Explanation
 (section 3.2).
 There are also attempts to find an *a priori* solution based
on probabilistic formulations of inductive inference, though many now
think that a purely *a priori* argument cannot be found because
there are empirical assumptions involved (sections
 3.3
 -3.5).


### 3.1 Synthetic *a priori*



As we have seen in
 section 1,
 Hume takes demonstrative arguments to have conclusions which are
"relations of ideas", whereas "probable" or
"moral" arguments have conclusions which are
"matters of fact". Hume's distinction between
"relations of ideas" and "matters of fact"
anticipates the distinction drawn by Kant between
"analytic" and "synthetic" propositions (Kant
1781). A classic example of an analytic proposition is
"Bachelors are unmarried men", and a synthetic proposition
is "My bike tyre is flat". For Hume, demonstrative
arguments, which are based on *a priori* reasoning, can
establish only relations of ideas, or analytic propositions. The
association between a prioricity and analyticity underpins
 premise P3,
 which states that a demonstrative argument establishes a conclusion
whose negation is a contradiction.



One possible response to Hume's problem is to deny
 premise P3,
 by allowing the possibility that *a priori* reasoning could
give rise to synthetic propositions. Kant famously argued in response
to Hume that such synthetic *a priori* knowledge is possible
(Kant 1781, 1783). He does this by a kind of reversal of the
empiricist programme espoused by Hume. Whereas Hume tried to
understand how the concept of a causal or necessary connection could
be based on experience, Kant argued instead that experience only comes
about through the concepts or "categories" of the
understanding. On his view, one can gain *a priori* knowledge
of these concepts, including the concept of causation, by a
transcendental argument concerning the necessary preconditions of
experience. A more detailed account of Kant's response to Hume
can be found in de Pierris and Friedman 2013.


### 3.2 The Nomological-Explanatory solution



The "Nomological-explanatory" solution, which has been put
forward by Armstrong, BonJour and Foster (Armstrong 1983; BonJour
1998; Foster 2004) appeals to the principle of Inference to the Best
Explanation (IBE). According to IBE, we should infer that the
hypothesis which provides the best explanation of the evidence is
probably true. Proponents of the Nomological-Explanatory approach take
Inference to the Best Explanation to be a mode of inference which is
distinct from the type of "extrapolative" inductive
inference that Hume was trying to justify. They also regard it as a
type of inference which although non-deductive, is justified *a
priori*. For example, Armstrong says "To infer to the best
explanation is part of what it is to be rational. If that is not
rational, what is?" (Armstrong 1983: 59).



The *a priori* justification is taken to proceed in two steps.
First, it is argued that we should recognize that certain observed
regularities require an explanation in terms of some underlying law.
For example, if a coin persistently lands heads on repeated tosses,
then it becomes increasingly implausible that this occurred just
because of "chance". Rather, we should infer to the better
explanation that the coin has a certain bias. Saying that the coin
lands heads not only for the observed cases, but also for the
unobserved cases, does not provide an explanation of the observed
regularity. Thus, mere Humean constant conjunction is not sufficient.
What is needed for an explanation is a "non-Humean,
metaphysically robust conception of objective regularity"
(BonJour 1998), which is thought of as involving actual natural
necessity (Armstrong 1983; Foster 2004).



Once it has been established that there must be some metaphysically
robust explanation of the observed regularity, the second step is to
argue that out of all possible metaphysically robust explanations, the
"straight" inductive explanation is the best one, where
the straight explanation extrapolates the observed frequency to the
wider population. For example, given that a coin has some objective
chance of landing heads, the best explanation of the fact that \(m/n\)
heads have been so far observed, is that the objective chance of the
coin landing heads is \(m/n\). And this objective chance determines
what happens not only in observed cases but also in unobserved
cases.



The Nomological-Explanatory solution relies on taking IBE as a
rational, *a priori* form of inference which is distinct from
inductive inferences like inference *I*. However, one might
alternatively view inductive inferences as a special case of IBE
(Harman 1968), or take IBE to be merely an alternative way of
characterizing inductive inference (Henderson 2014). If either of
these views is right, IBE does not have the necessary independence
from inductive inference to provide a non-circular justification of
it.



One may also object to the Nomological-Explanatory approach on the
grounds that regularities do not necessarily require an explanation in
terms of necessary connections or robust metaphysical laws. The
viability of the approach also depends on the tenability of a
non-Humean conception of laws. There have been several serious
attempts to develop such an account (Armstrong 1983; Tooley 1977;
Dretske 1977), but also much criticism (see J. Carroll 2016).



Another critical objection is that the Nomological-Explanatory
solution simply begs the question, even if it is taken to be
legitimate to make use of IBE in the justification of induction. In
the first step of the argument we infer to a law or regularity which
extends beyond the spatio-temporal region in which observations have
been thus far made, in order to predict what will happen in the
future. But why could a law that only applies to the observed
spatio-temporal region not be an equally good explanation? The main
reply seems to be that we can see *a priori* that laws with
temporal or spatial restrictions would be less good explanations.
Foster argues that the reason is that this would introduce more
mysteries:



> 
> 
> For it seems to me that a law whose scope is restricted to some
> particular period is more mysterious, inherently more puzzling, than
> one which is temporally universal. (Foster 2004)
> 
> 
> 


### 3.3 Bayesian solution



Another way in which one can try to construct an *a priori*
argument that the premises of an inductive inference make its
conclusion probable, is to make use of the formalism of probability
theory itself. At the time Hume wrote, probabilities were used to
analyze games of chance. And in general, they were used to address the
problem of what we would expect to see, given that a certain cause was
known to be operative. This is the so-called problem of "direct
inference". However, the problem of induction concerns the
"inverse" problem of determining the cause or general
hypothesis, given particular observations.



One of the first and most important methods for tackling the
"inverse" problem using probabilities was developed by
Thomas Bayes. Bayes's essay containing the main results was
published after his death in 1764 (Bayes 1764). However, it is
possible that the work was done significantly earlier and was in fact
written in direct response to the publication of Hume's Enquiry
in 1748 (see Zabell 1989: 290-93, for discussion of what is
known about the history).



We will illustrate the Bayesian method using the problem of drawing
balls from an urn. Suppose that we have an urn which contains white
and black balls in an unknown proportion. We draw a sample of balls
from the urn by removing a ball, noting its color, and then putting it
back before drawing again.



Consider first the problem of direct inference. Given the proportion
of white balls in the urn, what is the probability of various outcomes
for a sample of observations of a given size? Suppose the proportion
of white balls in the urn is \(\theta = 0.6\). The probability of
drawing one white ball in a sample of one is then \(p(W; \theta = 0.6)
= 0.6\). We can also compute the probability for other outcomes, such
as drawing two white balls in a sample of two, using the rules of the
probability calculus (see section 1 of Hajek 2011). Generally,
the probability that \(n\_w\) white balls are drawn in a sample of size
*N*, is given by the binomial distribution:

 
\[ p(n\_w;\theta=x) = \left(\begin{matrix}N\\
n\_w
\end{matrix}\right) x^{n\_w} (1-x)^{(N-n\_w)} \]


This is a specific example of a "sampling distribution",
\(p(E\mid H)\), which gives the probability of certain evidence
*E* in a sample, on the assumption that a certain hypothesis
*H* is true. Calculation of the sampling distribution can in
general be done *a priori*, given the rules of the probability
calculus.



However, the problem of induction is the inverse problem. We want to
infer not what the sample will be like, with a known hypothesis,
rather we want to infer a hypothesis about the general situation or
population, based on the observation of a limited sample. The
probabilities of the candidate hypotheses can then be used to inform
predictions about further observations. In the case of the urn, for
example, we want to know what the observation of a particular sample
frequency of white balls, \(\frac{n\_w}{N}\), tells us about
\(\theta\), the proportion of white balls in the urn.



The idea of the Bayesian approach is to assign probabilities not only
to the events which constitute evidence, but also to hypotheses. One
starts with a "prior probability" distribution over the
relevant hypotheses \(p(H)\). On learning some evidence *E*,
the Bayesian updates the prior \(p(H)\) to the conditional probability
\(p(H\mid E)\). This update rule is called the "rule of
conditionalisation". The conditional probability \(p(H\mid E)\)
is known as the "posterior probability", and is calculated
using Bayes' rule:

 
\[ p(H\mid E) = \frac{p(E\mid H) p(H)}{p(E)} \]


Here the sampling distribution can be taken to be a conditional
probability \(p(E\mid H)\), which is known as the
"likelihood" of the hypothesis *H* on evidence
*E*.



One can then go on to compute the predictive distribution for as yet
unobserved data \(E'\), given observations *E*. The predictive
distribution in a Bayesian approach is given by

 
\[ p(E'\mid E) = \sum\_{H} p(E'\mid H) p(H\mid E) \]


where the sum becomes an integral in cases where *H* is a
continuous variable.



For the urn example, we can compute the posterior probability
\(p(\theta\mid n\_w)\) using Bayes' rule, and the likelihood
given by the binomial distribution above. In order to do so, we also
need to assign a prior probability distribution to the parameter
\(\theta\). One natural choice, which was made early on by Bayes
himself and by Laplace, is to put a uniform prior over the parameter
\(\theta\). Bayes' own rationale for this choice was that then
if you work out the probability of each value for the number of whites
in the sample based only on the prior, before any data is observed,
all those probabilities are equal. Laplace had a different
justification, based on the Principle of Indifference. This principle
states that if you don't have any reason to favor one hypothesis
over another, you should assign them all equal probabilities.



With the choice of uniform prior, the posterior probability and
predictive distribution can be calculated. It turns out that the
probability that the next ball will be white, given that \(n\_w\) of
*N* draws were white, is given by

 
\[ p(w\mid n\_w) = \frac{n\_w + 1}{N+2} \]


This is Laplace's famous "rule of succession"
(1814). Suppose on the basis of observing 90 white balls out of 100,
we calculate by the rule of succession that the probability of the
next ball being white is \(91/102=0.89\). It is quite conceivable that
the next ball might be black. Even in the case, where all 100 balls
have been white, so that the probability of the next ball being white
is 0.99, there is still a small probability that the next ball is not
white. What the probabilistic reasoning supplies then is not an
argument to the conclusion that the next ball will be a certain color,
but an argument to the conclusion that certain future observations are
very *likely* given what has been observed in the past.



Overall, the Bayes-Laplace argument in the urn case provides an
example of how probabilistic reasoning can take us from evidence about
observations in the past to a prediction for how likely certain future
observations are. The question is what kind of solution, if any, this
type of calculation provides to the problem of induction. At first
sight, since it is just a mathematical calculation, it looks as though
it does indeed provide an *a priori* argument from the premises
of an inductive inference to the proposition that a certain conclusion
is probable.



However, in order to establish this definitively, one would need to
argue that all the components and assumptions of the argument are
*a priori* and this requires further examination of at least
three important issues.



First, the Bayes-Laplace argument relies on the rules of the
probability calculus. What is the status of these rules? Does
following them amount to *a priori* reasoning? The answer to
this depends in part on how probability itself is interpreted. Broadly
speaking, there are prominent interpretations of probability according
to which the rules plausibly have *a priori* status and could
form the basis of a demonstrative argument. These include the
classical interpretation originally developed by Laplace (1814), the
logical interpretation (Keynes (1921), Johnson (1921), Jeffreys
(1939), Carnap (1950), Cox (1946, 1961), and the subjectivist
interpretation of Ramsey (1926), Savage (1954), and de Finetti (1964).
Attempts to argue for a probabilistic *a priori* solution to
the problem of induction have been primarily associated with these
interpretations.



Secondly, in the case of the urn, the Bayes-Laplace argument is based
on a particular probabilistic model--the binomial model. This
involves the assumption that there is a parameter describing an
unknown proportion \(\theta\) of balls in the urn, and that the data
amounts to independent draws from a distribution over that parameter.
What is the basis of these assumptions? Do they generalize to other
cases beyond the actual urn case--i.e., can we see observations
in general as analogous to draws from an "Urn of Nature"?
There has been a persistent worry that these types of assumptions,
while reasonable when applied to the case of drawing balls from an
urn, will not hold for other cases of inductive inference. Thus, the
probabilistic solution to the problem of induction might be of
relatively limited scope. At the least, there are some assumptions
going into the choice of model here that need to be made explicit.
Arguably the choice of model introduces empirical assumptions, which
would mean that the probabilistic solution is not an *a priori*
one.



Thirdly, the Bayes-Laplace argument relies on a particular choice of
prior probability distribution. What is the status of this assignment,
and can it be based on *a priori* principles? Historically, the
Bayes-Laplace choice of a uniform prior, as well as the whole concept
of classical probability, relied on the Principle of Indifference.
This principle has been regarded by many as an *a priori*
principle. However, it has also been subjected to much criticism on
the grounds that it can give rise to inconsistent probability
assignments (Bertrand 1888; Borel 1909; Keynes 1921). Such
inconsistencies are produced by there being more than one way to carve
up the space of alternatives, and different choices give rise to
conflicting probability assignments. One attempt to rescue the
Principle of Indifference has been to appeal to explanationism, and
argue that the principle should be applied only to the carving of the
space at "the most explanatorily basic level", where this
level is identified according to an *a priori* notion of
explanatory priority (Huemer 2009).



The quest for an *a priori* argument for the assignment of the
prior has been largely abandoned. For many, the subjectivist
foundations developed by Ramsey, de Finetti and Savage provide a more
satisfactory basis for understanding probability. From this point of
view, it is a mistake to try to introduce any further *a
priori* constraints on the probabilities beyond those dictated by
the probability rules themselves. Rather the assignment of priors may
reflect personal opinions or background knowledge, and no prior is
*a priori* an unreasonable choice.



So far, we have considered probabilistic arguments which place
probabilities over hypotheses in a hypothesis space as well as
observations. There is also a tradition of attempts to determine what
probability distributions we should have, given certain observations,
from the starting point of a joint probability distribution over all
the observable variables. One may then postulate axioms directly on
this distribution over observables, and examine the consequences for
the predictive distribution. Much of the development of inductive
logic, including the influential programme by Carnap, proceeded in
this manner (Carnap 1950, 1952).



This approach helps to clarify the role of the assumptions behind
probabilistic models. One assumption that one can make about the
observations is that they are "exchangeable". This means
that the joint distribution of the random variables is invariant under
permutations. Informally, this means that the order of the
observations does not affect the probability. For instance, in the urn
case, this would mean that drawing first a white ball and then a black
ball is just as probable as first drawing a black and then a white. De
Finetti proved a general representation theorem that if the joint
probability distribution of an infinite sequence of random variables
is assumed to be exchangeable, then it can be written as a mixture of
distribution functions from each of which the data behave as if they
are independent random draws (de Finetti 1964). In the case of the urn
example, the theorem shows that it is *as if* the data are
independent random draws from a binomial distribution over a parameter
\(\theta\), which itself has a prior probability distribution.



The assumption of exchangeability may be seen as a natural
formalization of Hume's assumption that the past resembles the
future. This is intuitive because assuming exchangeability means
thinking that the order of observations, both past and future, does
not matter to the probability assignments.



However, the development of the programme of inductive logic revealed
that many generalizations are possible. For example, Johnson proposed
to assume an axiom he called the "sufficientness
postulate". This states that outcomes can be of a number of
different types, and that the conditional probability that the next
outcome is of type *i* depends only on the number of previous
trials and the number of previous outcomes of type *i* (Johnson
1932). Assuming the sufficientness postulate for three or more types
gives rise to a general predictive distribution corresponding to
Carnap's "continuum of inductive methods" (Carnap
1952). This predictive distribution takes the form:

 
\[ p(i\mid N\_1,N\_2,\ldots N\_t)= \frac{N\_i + k}{N\_1 +N\_2 + \cdots + N\_t + kt} \]


for some positive number *k*. This reduces to Laplace's
rule of succession when \(t=2\) and \(k=1\).



Generalizations of the notion of exchangeability, such as
"partial exchangeability" and "Markov
exchangeability", have been explored, and these may be thought
of as forms of symmetry assumption (Zabell 1988; Skyrms 2012). As less
restrictive axioms on the probabilities for observables are assumed,
the result is that there is no longer a unique result for the
probability of a prediction, but rather a whole class of possible
probabilities, mapped out by a generalized rule of succession such as
the above. Therefore, in this tradition, as in the Bayes-Laplace
approach, we have moved away from producing an argument which produces
a unique a priori probabilistic answer to Hume's problem.



One might think then that the assignment of the prior, or the relevant
corresponding postulates on the observable probability distribution,
is precisely where empirical assumptions enter into inductive
inferences. The probabilistic calculations are empirical arguments,
rather than *a priori* ones. If this is correct, then the
probabilistic framework has not in the end provided an *a
priori* solution to the problem of induction, but it has rather
allowed us to clarify what could be meant by Hume's claim that
inductive inferences rely on the Uniformity Principle.


### 3.4 Partial solutions



Some think that although the problem of induction is not solved, there
is in some sense a partial solution, which has been called a
"logical solution". Howson, for example, argues that
"*Inductive reasoning is justified to the extent that it is
sound, given appropriate premises*" (Howson 2000: 239, his
emphasis). According to this view, there is no getting away from an
empirical premise for inductive inferences, but we might still think
of Bayesian conditioning as functioning like a kind of logic or
"consistency constraint" which "generates
predictions from the assumptions and observations together"
(Romeijn 2004: 360). Once we have an empirical assumption,
instantiated in the prior probability, and the observations, Bayesian
conditioning tells us what the resulting predictive probability
distribution should be.



The idea of a partial solution also arises in the context of the
learning theory that grounds contemporary machine learning. Machine
learning is a field in computer science concerned with algorithms that
learn from experience. Examples are algorithms which can be trained to
recognise or classify patterns in data. Learning theory concerns
itself with finding mathematical theorems which guarantee the
performance of algorithms which are in practical use. In this domain,
there is a well-known finding that learning algorithms are only
effective if they have 'inductive bias' -- that is, if
they make some a priori assumptions about the domain they are employed
upon (Mitchell 1997).



The idea is also given formal expression in the so-called
'No-Free-Lunch theorems' (Wolpert 1992, 1996, 1997). These
can be interpreted as versions of the argument in Hume's first
fork since they establish that there can be no contradiction in the
algorithm not performing well, since there are *a priori*
possible situations in which it does not (Sterkenburg and
Grunwald 2021:9992). Given Hume's premise
 P3,
 this rules out a demonstrative argument for its good performance.



Premise
 P3
 can perhaps be challenged on the grounds that *a priori*
justifications can also be given for contingent propositions. Even
though an inductive inference can fail in some possible situations, it
could still be reasonable to form an expectation of reliability if we
spread our credence equally over all the possibilities and have reason
to think (or at least no reason to doubt) that the cases where
inductive inference is unreliable require a 'very specific
arrangement of things' and thus form a small fraction of the
total space of possibilities (White 2015). The No-Free-Lunch theorems
make difficulties for this approach since they show that if we put a
uniform distribution over all logically possible sequences of future
events, any learning algorithm is expected to have a generalisation
error of 1/2, and hence to do no better than guessing at random
(Schurz 2021b).



The No-Free-Lunch theorems may be seen as fundamental limitations on
justifying learning algorithms when these algorithms are seen as
'purely data-driven' -- that is as mappings from possible
data to conclusions. However, learning algorithms may also be
conceived as functions not only of input data, but also of a
particular model (Sterkenburg and Grunwald 2021). For example,
the Bayesian 'algorithm' gives a universal recipe for
taking a particular model and prior and updating on the data. A number
of theorems in learning theory provide general guarantees for the
performance of such recipes. For instance, there are theorems which
guarantee convergence of the Bayesian algorithm (Ghosal, Ghosh and van
der Vaart 2000, Ghosal, Lember and van der Vaart 2008). In each
instantiation, this convergence is relative to a particular specific
prior. Thus, although the considerations first raised by Hume, and
later instantiated in the No-Free-Lunch theorems, preclude any
universal model-independent justification for learning algorithms, it
does not rule out partial justifications in the form of such general a
priori 'model-relative' learning guarantees (Sterkenburg
and Grunwald 2021).


### 3.5 The combinatorial approach



An alternative attempt to use probabilistic reasoning to produce an
*a priori* justification for inductive inferences is the
so-called "combinatorial" solution. This was first put
forward by Donald C. Williams (1947) and later developed by David
Stove (1986).



Like the Bayes-Laplace argument, the solution relies heavily on the
idea that straightforward *a priori* calculations can be done
in a "direct inference" from population to sample. As we
have seen, given a certain population frequency, the probability of
getting different frequencies in a sample can be calculated
straightforwardly based on the rules of the probability calculus. The
Bayes-Laplace argument relied on inverting the probability
distribution using Bayes' rule to get from the sampling
distribution to the posterior distribution. Williams instead proposes
that the inverse inference may be based on a certain logical
syllogism: the proportional (or statistical) syllogism.



The proportional, or statistical syllogism, is the following:


1. Of all the things that are *M*, \(m/n\) are
*P*.
2. *a* is an *M*



Therefore, *a* is *P*, with probability \(m/n\).



For example, if 90% of rabbits in a population are white and we
observe a rabbit *a*, then the proportional syllogism says that
we infer that *a* is white with a probability of 90%. Williams
argues that the proportional syllogism is a non-deductive logical
syllogism, which effectively interpolates between the syllogism for
entailment


1. All *M*s are *P*
2. *a* is an *M*



Therefore, *a* is *P*.



And the syllogism for contradiction


1. No *M* is *P*
2. *a* is *M*



Therefore, *a* is not *P*.



This syllogism can be combined with an observation about the behavior
of increasingly large samples. From calculations of the sampling
distribution, it can be shown that as the sample size increases, the
probability that the sample frequency is in a range which closely
approximates the population frequency also increases. In fact,
Bernoulli's law of large numbers states that the probability
that the sample frequency approximates the population frequency tends
to one as the sample size goes to infinity. Williams argues that such
results support a "general over-all premise, common to all
inductions, that samples 'match' their populations"
(Williams 1947: 78).



We can then apply the proportional syllogism to samples from a
population, to get the following argument:


1. Most samples match their population
2. *S* is a sample.



Therefore, *S* matches its population, with high
probability.



This is an instance of the proportional syllogism, and it uses the
general result about samples matching populations as the first major
premise.



The next step is to argue that if we observe that the sample contains
a proportion of \(m/n\) *F*s, then we can conclude that since
this sample with high probability matches its population, the
population, with high probability, has a population frequency that
approximates the sample frequency \(m/n\). Both Williams and Stove
claim that this amounts to a logical *a priori* solution to the
problem of induction.



A number of authors have expressed the view that the Williams-Stove
argument is only valid if the sample *S* is drawn randomly from
the population of possible samples--i.e., that any sample is as
likely to be drawn as any other (Brown 1987; Will 1948; Giaquinto
1987). Sometimes this is presented as an objection to the application
of the proportional syllogism. The claim is that the proportional
syllogism is only valid if *a* is drawn randomly from the
population of *M*s. However, the response has been that there
is no need to know that the sample is randomly drawn in order to apply
the syllogism (Maher 1996; Campbell 2001; Campbell & Franklin
2004). Certainly if you have reason to think that your sampling
procedure is more likely to draw certain individuals than
others--for example, if you know that you are in a certain
location where there are more of a certain type--then you should
not apply the proportional syllogism. But if you have no such reasons,
the defenders claim, it is quite rational to apply it. Certainly it is
always possible that you draw an unrepresentative sample--meaning
one of the few samples in which the sample frequency does not match
the population frequency--but this is why the conclusion is only
probable and not certain.



The more problematic step in the argument is the final step, which
takes us from the claim that samples match their populations with high
probability to the claim that having seen a particular sample
frequency, the population from which the sample is drawn has frequency
close to the sample frequency with high probability. The problem here
is a subtle shift in what is meant by "high probability",
which has formed the basis of a common misreading of
Bernouilli's theorem. Hacking (1975: 156-59) puts the
point in the following terms. Bernouilli's theorem licenses the
claim that much more often than not, a small interval around the
sample frequency will include the true population frequency. In other
words, it is highly probable in the sense of "usually
right" to say that the sample matches its population. But this
does not imply that the proposition that a small interval around the
sample will contain the true population frequency is highly probable
in the sense of "credible on each occasion of use". This
would mean that for any given sample, it is highly credible that the
sample matches its population. It is quite compatible with the claim
that it is "usually right" that the sample matches its
population to say that there are some samples which do not match their
populations at all. Thus one cannot conclude from Bernouilli's
theorem that for any given sample frequency, we should assign high
probability to the proposition that a small interval around the sample
frequency will contain the true population frequency. But this is
exactly the slide that Williams makes in the final step of his
argument. Maher (1996) argues in a similar fashion that the last step
of the Williams-Stove argument is fallacious. In fact, if one wants to
draw conclusions about the probability of the population frequency
given the sample frequency, the proper way to do so is by using the
Bayesian method described in the previous section. But, as we there
saw, this requires the assignment of prior probabilities, and this
explains why many people have thought that the combinatorial solution
somehow illicitly presupposed an assumption like the principle of
indifference. The Williams-Stove argument does not in fact give us an
alternative way of inverting the probabilities which somehow bypasses
all the issues that Bayesians have faced.


## 4. Tackling the Second Horn of Hume's Dilemma



So far we have considered ways in which the first horn of Hume's
dilemma might be tackled. But it is of course also possible to take on
the second horn instead.



One may argue that a probable argument would not, despite what Hume
says, be circular in a problematic way (we consider responses of this
kind in
 section 4.1).
 Or, one might attempt to argue that probable arguments are not
circular at all
 (section 4.2).


### 4.1 Inductive Justifications of Induction



One way to tackle the second horn of Hume's dilemma is to reject
 premise P6,
 which rules out circular arguments. Some have argued that certain
kinds of circular arguments would provide an acceptable justification
for the inductive inference. Since the justification would then itself
be an inductive one, this approach is often referred to as an
"inductive justification of induction".



First we should examine how exactly the Humean circularity supposedly
arises. Take the simple case of enumerative inductive inference that
follows the following pattern (*X*):




Most observed *F*s have been *G*s



Therefore: Most *F*s are *G*s.




Hume claims that such arguments presuppose the Uniformity Principle
(UP). According to premises
 P7
 and
 P8,
 this supposition also needs to be supported by an argument in order
that the inductive inference be justified. A natural idea is that we
can argue for the Uniformity Principle on the grounds that "it
works". We know that it works, because past instances of
arguments which relied upon it were found to be successful. This alone
however is not sufficient unless we have reason to think that such
arguments will also be successful in the future. That claim must
itself be supported by an inductive argument (*S*):




Most arguments of form *X* that rely on UP have succeeded in
the past.



Therefore, most arguments of form *X* that rely on UP
succeed.




But this argument itself depends on the UP, which is the very
supposition which we were trying to justify.



As we have seen in
 section 2,
 some reject Hume's claim that all inductive inferences
presuppose the UP. However, the argument that basing the justification
of the inductive inference on a probable argument would result in
circularity need not rely on this claim. The circularity concern can
be framed more generally. If argument *S* relies on
*something* which is already presupposed in inference
*X*, then argument *S* cannot be used to justify
inference *X*. The question though is what precisely the
something is.



Some authors have argued that in fact *S* does not rely on any
premise or even presupposition that would require us to already know
the conclusion of *X*. *S* is then not a "premise
circular" argument. Rather, they claim, it is
"rule-circular"--it relies on a rule of inference in
order to reach the conclusion that that very rule is reliable. Suppose
we adopt the rule *R* which says that when it is observed that
most *F*s are *G*s, we should infer that most
*F*s are *G*s. Then inference *X* relies on rule
*R*. We want to show that rule *R* is reliable. We could
appeal to the fact that *R* worked in the past, and so, by an
inductive argument, it will also work in the future. Call this
argument *S*\*:




Most inferences following rule *R* have been successful



Therefore, most inferences following *R* are successful.




Since this argument itself uses rule *R*, using it to establish
that *R* is reliable is rule-circular.



Some authors have then argued that although premise-circularity is
vicious, rule-circularity is not (Cleve 1984; Papineau 1992). One
reason for thinking rule-circularity is not vicious would be if it is
not necessary to know or even justifiably believe that rule *R*
is reliable in order to move to a justified conclusion using the rule.
This is a claim made by externalists about justification (Cleve 1984).
They say that as long as *R* is *in fact* reliable, one
can form a justified belief in the conclusion of an argument relying
on *R*, as long as one has justified belief in the
premises.



If one is not persuaded by the externalist claim, one might attempt to
argue that rule circularity is benign in a different fashion. For
example, the requirement that a rule be shown to be reliable without
any rule-circularity might appear unreasonable when the rule is of a
very fundamental nature. As Lange puts it:



> 
> 
> It might be suggested that although a circular argument is ordinarily
> unable to justify its conclusion, a circular argument is acceptable in
> the case of justifying a fundamental form of reasoning. After all,
> there is nowhere more basic to turn, so all that we can reasonably
> demand of a fundamental form of reasoning is that it endorse itself.
> (Lange 2011: 56)
> 
> 
> 



Proponents of this point of view point out that even deductive
inference cannot be justified deductively. Consider Lewis
Carroll's dialogue between Achilles and the Tortoise (Carroll
1895). Achilles is arguing with a Tortoise who refuses to perform
*modus ponens*. The Tortoise accepts the premise that
*p*, and the premise that *p* implies *q* but he
will not accept *q*. How can Achilles convince him? He manages
to persuade him to accept another premise, namely "if *p*
and *p* implies *q*, then *q*". But the
Tortoise is still not prepared to infer to *q*. Achilles goes
on adding more premises of the same kind, but to no avail. It appears
then that *modus ponens* cannot be justified to someone who is
not already prepared to use that rule.



It might seem odd if premise circularity were vicious, and rule
circularity were not, given that there appears to be an easy
interchange between rules and premises. After all, a rule can always,
as in the Lewis Carroll story, be added as a premise to the argument.
But what the Carroll story also appears to indicate is that there is
indeed a fundamental difference between being prepared to accept a
premise stating a rule (the Tortoise is happy to do this), and being
prepared to use that rule (this is what the Tortoise refuses to
do).



Suppose that we grant that an inductive argument such as *S*
(or *S*\*) can support an inductive inference *X* without
vicious circularity. Still, a possible objection is that the argument
simply does not provide a full justification of *X*. After all,
less sane inference rules such as counterinduction can support
themselves in a similar fashion. The counterinductive rule is CI:




Most observed *A*s are *B*s.



Therefore, it is not the case that most *A*s are
*B*s.




Consider then the following argument CI\*:




Most CI arguments have been unsuccessful



Therefore, it is not the case that most CI arguments are unsuccessful,
i.e., many CI arguments are successful.




This argument therefore establishes the reliability of CI in a
rule-circular fashion (see Salmon 1963).



Argument *S* can be used to support inference *X*, but
only for someone who is already prepared to infer inductively by using
*S*. It cannot convince a skeptic who is not prepared to rely
upon that rule in the first place. One might think then that the
argument is simply not achieving very much.



The response to these concerns is that, as Papineau puts it, the
argument is "not *supposed* to do very much"
(Papineau 1992: 18). The fact that a counterinductivist counterpart of
the argument exists is true, but irrelevant. It is conceded that the
argument cannot persuade either a counterinductivist, or a skeptic.
Nonetheless, proponents of the inductive justification maintain that
there is still some added value in showing that inductive inferences
are reliable, even when we already accept that there is nothing
problematic about them. The inductive justification of induction
provides a kind of important consistency check on our existing
beliefs.


### 4.2 No Rules



It is possible to go even further in an attempt to dismantle the
Humean circularity. Maybe inductive inferences do not even have a rule
in common. What if every inductive inference is essentially unique?
This can be seen as rejecting Hume's premise
 P5.
 Okasha, for example, argues that Hume's circularity problem can
be evaded if there are "no rules" behind induction (Okasha
2005a,b). Norton puts forward the similar idea that all inductive
inferences are material, and have nothing formal in common (Norton
2003, 2010, 2021).



Proponents of such views have attacked Hume's claim that there
is a UP on which all inductive inferences are based. There have long
been complaints about the vagueness of the Uniformity Principle
(Salmon 1953). The future only resembles the past in some respects,
but not others. Suppose that on all my birthdays so far, I have been
under 40 years old. This does not give me a reason to expect that I
will be under 40 years old on my next birthday. There seems then to be
a major lacuna in Hume's account. He might have explained or
described how we draw an inductive inference, on the assumption that
it is one we *can* draw. But he leaves untouched the question
of how we distinguish between cases where we extrapolate a regularity
legitimately, regarding it as a law, and cases where we do not.



Nelson Goodman is often seen as having made this point in a
particularly vivid form with his "new riddle of induction"
(Goodman 1955: 59-83). Suppose we define a predicate
"grue" in the following way. An object is
"grue" when it is green if observed before time *t*
and blue otherwise. Goodman considers a thought experiment in which we
observe a bunch of green emeralds before time *t*. We could
describe our results by saying all the observed emeralds are green.
Using a simple enumerative inductive schema, we could infer from the
result that all observed emeralds are green, that all emeralds are
green. But equally, we could describe the same results by saying that
all observed emeralds are grue. Then using the same schema, we could
infer from the result that all observed emeralds are grue, that all
emeralds are grue. In the first case, we expect an emerald observed
after time *t* to be green, whereas in the second, we expect it
to be blue. Thus the two predictions are incompatible. Goodman claims
that what Hume omitted to do was to give any explanation for why we
project predicates like "green", but not predicates like
"grue". This is the "new riddle", which is
often taken to be a further problem of induction that Hume did not
address.



One moral that could be taken from Goodman is that there is not one
general Uniformity Principle that all probable arguments rely upon
(Sober 1988; Norton 2003; Okasha 2001, 2005a,b, Jackson 2019). Rather
each inductive inference presupposes some more specific empirical
presupposition. A particular inductive inference depends on some
specific way in which the future resembles the past. It can then be
justified by another inductive inference which depends on some quite
different empirical claim. This will in turn need to be
justified--by yet another inductive inference. The nature of
Hume's problem in the second horn is thus transformed. There is
no circularity. Rather there is a regress of inductive justifications,
each relying on their own empirical presuppositions (Sober 1988;
Norton 2003; Okasha 2001, 2005a,b).



One way to put this point is to say that Hume's argument rests
on a quantifier shift fallacy (Sober 1988; Okasha 2005a). Hume says
that there exists a general presupposition for all inductive
inferences, whereas he should have said that for each inductive
inference, there is some presupposition. Different inductive
inferences then rest on different empirical presuppositions, and the
problem of circularity is evaded.



What will then be the consequence of supposing that Hume's
problem should indeed have been a regress, rather than a circularity?
Here different opinions are possible. On the one hand, one might think
that a regress still leads to a skeptical conclusion (Schurz and Thorn
2020). So although the exact form in which Hume stated his problem was
not correct, the conclusion is not substantially different (Sober
1988). Another possibility is that the transformation mitigates or
even removes the skeptical problem. For example, Norton argues that
the upshot is a dissolution of the problem of induction, since the
regress of justifications benignly terminates (Norton 2003). And
Okasha more mildly suggests that even if the regress is infinite,
"Perhaps infinite regresses are less bad than vicious circles
after all" (Okasha 2005b: 253).



Any dissolution of Hume's circularity does not depend only on
arguing that the UP should be replaced by empirical presuppositions
which are specific to each inductive inference. It is also necessary
to establish that inductive inferences share no common
rules--otherwise there will still be at least some
rule-circularity. Okasha suggests that the Bayesian model of
belief-updating is an illustration how induction can be characterized
in a rule-free way, but this is problematic, since in this model all
inductive inferences still share the common rule of Bayesian
conditionalisation. Norton's material theory of induction
postulates a rule-free characterization of induction, but it is not
clear whether it really can avoid any role for general rules
(Achinstein 2010, Kelly 2010, Worrall 2010).


## 5. Alternative Conceptions of Justification



Hume is usually read as delivering a negative verdict on the
possibility of justifying inference *I*, via a premise such as
 P8,
 though as we have seen in section
 section 2,
 some have questioned whether Hume is best interpreted as drawing a
conclusion about justification of inference *I* at all. In this
section we examine approaches which question in different ways whether
 premise P8
 really does give a valid necessary condition for justification of
inference *I* and propose various alternative conceptions of
justification.


### 5.1 Postulates and Hinges



One approach has been to turn to general reflection on what is even
needed for justification of an inference in the first place. For
example, Wittgenstein raised doubts over whether it is even meaningful
to ask for the grounds for inductive inferences.



> 
> 
> If anyone said that information about the past could not convince him
> that something would happen in the future, I should not understand
> him. One might ask him: what do you expect to be told, then? What sort
> of information do you call a ground for such a belief? ... If
> these are not grounds, then what are grounds?--If you say these
> are not grounds, then you must surely be able to state what must be
> the case for us to have the right to say that there are grounds for
> our assumption.... (Wittgenstein 1953: 481)
> 
> 
> 



One might not, for instance, think that there even needs to be a chain
of reasoning in which each step or presupposition is supported by an
argument. Wittgenstein took it that there are some principles so
fundamental that they do not require support from any further
argument. They are the "hinges" on which enquiry
turns.



Out of Wittgenstein's ideas has developed a general notion of
"entitlement", which is a kind of rational warrant to hold
certain propositions which does not come with the same requirements as
"justification". Entitlement provides epistemic rights to
hold a proposition, without responsibilities to base the belief in it
on an argument. Crispin Wright (2004) has argued that there are
certain principles, including the Uniformity Principle, that we are
entitled in this sense to hold.



Some philosophers have set themselves the task of determining a set or
sets of postulates which form a plausible basis for inductive
inferences. Bertrand Russell, for example, argued that five postulates
lay at the root of inductive reasoning (Russell 1948). Arthur Burks,
on the other hand, proposed that the set of postulates is not unique,
but there may be multiple sets of postulates corresponding to
different inductive methods (Burks 1953, 1955).



The main objection to all these views is that they do not really solve
the problem of induction in a way that adequately secures the pillars
on which inductive inference stands. As Salmon puts it,
"admission of unjustified and unjustifiable postulates to deal
with the problem is tantamount to making scientific method a matter of
faith" (Salmon 1966: 48).


### 5.2 Ordinary Language Dissolution



Rather than allowing undefended empirical postulates to give normative
support to an inductive inference, one could instead argue for a
completely different conception of what is involved in justification.
Like Wittgenstein, later ordinary language philosophers, notably P.F.
Strawson, also questioned what exactly it means to ask for a
justification of inductive inferences (Strawson 1952). This has become
known as the "Ordinary language dissolution" of the
problem of induction.



Strawson points out that it could be meaningful to ask for a deductive
justification of inductive inferences. But it is not clear that this
is helpful since this is effectively "a demand that induction
shall be shown to be really a kind of deduction" (Strawson 1952:
230). Rather, Strawson says, when we ask about whether a particular
inductive inference is justified, we are typically judging whether it
conforms to our usual inductive standards. Suppose, he says, someone
has formed the belief by inductive inference that All
*f*'s are *g*. Strawson says that if that person
is asked for their grounds or reasons for holding that belief,



> 
> 
> I think it would be felt to be a satisfactory answer if he replied:
> "Well, in all my wide and varied experience I've come
> across innumerable cases of *f* and never a case of *f*
> which wasn't a case of *g*". In saying this, he is
> clearly claiming to have *inductive* support,
> *inductive* evidence, of a certain kind, for his belief.
> (Strawson 1952)
> 
> 
> 



That is just because inductive support, as it is usually understood,
simply consists of having observed many positive instances in a wide
variety of conditions.



In effect, this approach denies that producing a chain of reasoning is
a necessary condition for justification. Rather, an inductive
inference is justified if it conforms to the usual standards of
inductive justification. But, is there more to it? Might we not ask
what reason we have to rely on those inductive standards?



It surely makes sense to ask whether a particular inductive inference
is justified. But the answer to that is fairly straightforward.
Sometimes people have enough evidence for their conclusions and
sometimes they do not. Does it also make sense to ask about whether
inductive procedures generally are justified? Strawson draws the
analogy between asking whether a particular act is legal. We may
answer such a question, he says, by referring to the law of the
land.



> 
> 
> But it makes no sense to inquire in general whether the law of the
> land, the legal system as a whole, is or is not legal. For to what
> legal standards are we appealing? (Strawson 1952: 257)
> 
> 
> 



According to Strawson,



> 
> 
> It is an analytic proposition that it is reasonable to have a degree
> of belief in a statement which is proportional to the strength of the
> evidence in its favour; and it is an analytic proposition, though not
> a proposition of mathematics, that, other things being equal, the
> evidence for a generalisation is strong in proportion as the number of
> favourable instances, and the variety of circumstances in which they
> have been found, is great. So to ask whether it is reasonable to place
> reliance on inductive procedures is like asking whether it is
> reasonable to proportion the degree of one's convictions to the
> strength of the evidence. Doing this is what "being
> reasonable" *means* in such a context. (Strawson 1952:
> 256-57)
> 
> 
> 



Thus, according to this point of view, there is no further question to
ask about whether it is reasonable to rely on inductive
inferences.



The ordinary language philosophers do not explicitly argue against
Hume's
 premise P8.
 But effectively what they are doing is offering a whole different
story about what it would mean to be justified in believing the
conclusion of inductive inferences. What is needed is just conformity
to inductive standards, and there is no real meaning to asking for any
further justification for those.



The main objection to this view is that conformity to the usual
standards is insufficient to provide the needed justification. What we
need to know is whether belief in the conclusion of an inductive
inference is "epistemically reasonable or justified in the sense
that ...there is reason to think that it is likely to be
true" (BonJour 1998: 198). The problem Hume has raised is
whether, despite the fact that inductive inferences have tended to
produce true conclusions in the past, we have reason to think the
conclusion of an inductive inference we now make is likely to be true.
Arguably, establishing that an inductive inference is rational in the
sense that it follows inductive standards is not sufficient to
establish that its conclusion is likely to be true. In fact Strawson
allows that there is a question about whether "induction will
continue to be successful", which is distinct from the question
of whether induction is rational. This question he does take to hinge
on a "contingent, factual matter" (Strawson 1952: 262).
But if it is this question that concerned Hume, it is no answer to
establish that induction is rational, unless that claim is understood
to involve or imply that an inductive inference carried out according
to rational standards is likely to have a true conclusion.


### 5.3 Pragmatic vindication of induction



Another solution based on an alternative criterion for justification
is the "pragmatic" approach initiated by Reichenbach (1938
[2006]). Reichenbach did think Hume's argument unassailable, but
nonetheless he attempted to provide a weaker kind of justification for
induction. In order to emphasize the difference from the kind of
justification Hume sought, some have given it a different term and
refer to Reichenbach's solution as a "vindication",
rather than a justification of induction (Feigl 1950; Salmon
1963).



Reichenbach argued that it was not necessary for the justification of
inductive inference to show that its conclusion is true. Rather
"the proof of the truth of the conclusion is only a sufficient
condition for the justification of induction, not a necessary
condition" (Reichenbach 2006: 348). If it could be shown, he
says, that inductive inference is a necessary condition of success,
then even if we do not know that it will succeed, we still have some
reason to follow it. Reichenbach makes a comparison to the situation
where a man is suffering from a disease, and the physician says
"I do not know whether an operation will save the man, but if
there is any remedy, it is an operation" (Reichenbach 1938
[2006: 349]). This provides some kind of justification for operating
on the man, even if one does not know that the operation will
succeed.



In order to get a full account, of course, we need to say more about
what is meant for a method to have "success", or to
"work". Reichenbach thought that this should be defined in
relation to the aim of induction. This aim, he thought, is
"*to find series of events whose frequency of occurrence
converges towards a limit*" (1938 [2006: 350]).



Reichenbach applied his strategy to a general form of
"statistical induction" in which we observe the relative
frequency \(f\_n\) of a particular event in *n* observations and
then form expectations about the frequency that will arise when more
observations are made. The "inductive principle" then
states that if after a certain number of instances, an observed
frequency of \(m/n\) is observed, for any prolongation of the series
of observations, the frequency will continue to fall within a small
interval of \(m/n\). Hume's examples are special cases of this
principle, where the observed frequency is 1. For example, in
Hume's bread case, suppose bread was observed to nourish
*n* times out of *n* (i.e. an observed frequency of
100%), then according to the principle of induction, we expect that as
we observe more instances, the frequency of nourishing ones will
continue to be within a very small interval of 100%. Following this
inductive principle is also sometimes referred to as following the
"straight rule". The problem then is to justify the use of
this rule.



Reichenbach argued that even if Hume is right to think that we cannot
be justified in thinking for any particular application of the rule
that the conclusion is likely to be true, for the purposes of
practical action we do not need to establish this. We can instead
regard the inductive rule as resulting in a "posit", or
statement that we deal with as if it is true. We posit a certain
frequency *f* on the basis of our evidence, and this is like
making a wager or bet that the frequency is in fact *f*. One
strategy for positing frequencies is to follow the rule of
induction.



Reichenbach proposes that we can show that the rule of induction meets
his weaker justification condition. This does not require showing that
following the inductive principle will always work. It is possible
that the world is so disorderly that we cannot construct series with
any limits. In that case, neither the inductive principle, nor any
other method will succeed. But, he argues, if there is a limit, by
following the inductive principle we will eventually find it. There is
some element of a series of observations, beyond which the principle
of induction will lead to the true value of the limit. Although the
inductive rule may give quite wrong results early in the sequence, as
it follows chance fluctuations in the sample frequency, it is
guaranteed to eventually approximate the limiting frequency, if such a
limit exists. Therefore, the rule of induction is justified as an
instrument of positing because it is a method of which we know that if
it is possible to achieve the aim of inductive inference we shall do
so by means of this method (Reichenbach 1949: 475).



One might question whether Reichenbach has achieved his goal of
showing that following the inductive rule is a necessary condition of
success. In order to show that, one would also need to establish that
no other methods can also achieve the aim. But, as Reichenbach himself
recognises, many other rules of inference as well as the straight rule
may also converge on the limit (Salmon 1966: 53). In fact, any method
which converges asymptotically to the straight rule also does so. An
easily specified class of such rules are those which add to the
inductive rule a function \(c\_n\) in which the \(c\_n\) converge to
zero with increasing *n*.



Reichenbach makes two suggestions aimed at avoiding this problem. On
the one hand, he claims, since we have no real way to pick between
methods, we might as well just use the inductive rule since it is
"easier to handle, owing to its descriptive simplicity".
He also claims that the method which embodies the "smallest
risk" is following the inductive rule (Reichenbach 1938 [2006:
355-356]).



There is also the concern that there could be a completely different
kind of rule which converges on the limit. We can consider, for
example, the possibility of a soothsayer or psychic who is able to
predict future events reliably. Here Reichenbach argues that induction
is still necessary in such a case, because it has to be used to check
whether the other method works. It is only by using induction,
Reichenbach says, that we could recognise the reliability of the
alternative method, by examining its track record.



In assessing this argument, it is helpful to distinguish between
levels at which the principle of induction can be applied. Following
Skyrms (2000), we may distinguish between level 1, where candidate
methods are applied to ordinary events or individuals, and level 2,
where they are applied not to individuals or events, but to the
arguments on level 1. Let us refer to "object-induction"
when the inductive principle is applied at level 1, and
"meta-induction" when it is applied at level 2.
Reichenbach's response does not rule out the possibility that
another method might do better than object-induction at level 1. It
only shows that the success of that other method may be recognised by
a meta-induction at level 2 (Skyrms 2000). Nonetheless,
Reichenbach's thought was later picked up and developed into the
suggestion that a meta-inductivist who applies induction not only at
the object level to observations, but also to the success of
others' methods, might by those means be able to do as well
predictively as the alternative method (Schurz 2008; see
 section 5.5
 for more discussion of meta-induction).



Reichenbach's justification is generally taken to be a pragmatic
one, since though it does not supply knowledge of a future event, it
supplies a sufficient reason for action (Reichenbach 1949: 481). One
might question whether a pragmatic argument can really deliver an
all-purpose, general justification for following the inductive rule.
Surely a pragmatic solution should be sensitive to differences in
pay-offs that depend on the circumstances. For example, Reichenbach
offers the following analogue to his pragmatic justification:



> 
> 
> We may compare our situation to that of a man who wants to fish in an
> unexplored part of the sea. There is no one to tell him whether or not
> there are fish in this place. Shall he cast his net? Well, if he wants
> to fish in that place, I should advise him to cast the net, to take
> the chance at least. It is preferable to try even in uncertainty than
> not to try and be certain of getting nothing. (Reichenbach 1938 [2006:
> 362-363])
> 
> 
> 



As Lange points out, the argument here "presumes that there is
no cost to trying". In such a situation, "the fisherman
has everything to gain and nothing to lose by casting his net"
(Lange 2011: 77). But if there is some significant cost to making the
attempt, it may not be so clear that the most rational course of
action is to cast the net. Similarly, whether or not it would make
sense to adopt the policy of making no predictions, rather than the
policy of following the inductive rule, may depend on what the
practical penalties are for being wrong. A pragmatic solution may not
be capable of offering rationale for following the inductive rule
which is applicable in all circumstances.



Another question is whether Reichenbach has specified the aim of
induction too narrowly. Finding series of events whose frequency of
occurrence converges to a limit ties the vindication to the long-run,
while allowing essentially no constraint on what can be posited in the
short-run. Yet it is in the short run that inductive practice actually
occurs and where it really needs justification (BonJour 1998: 194;
Salmon 1966: 53).


### 5.4 Formal Learning Theory



Formal learning theory can be regarded as a kind of extension of the
Reichenbachian programme. It does not offer justifications for
inductive inferences in the sense of giving reasons why they should be
taken as likely to provide a true conclusion. Rather it offers a
"means-ends" epistemology -- it provides reasons for
following particular methods based on their optimality in achieving
certain desirable epistemic ends, even if there is no guarantee that
at any given stage of inquiry the results they produce are at all
close to the truth (Schulte 1999).



Formal learning theory is particularly concerned with showing that
methods are "logically reliable" in the sense that they
arrive at the truth given any sequence of data consistent with our
background knowledge (Kelly 1996). However, it goes further than this.
As we have just seen, one of the problems for Reichenbach was that
there are too many rules which converge in the limit to the true
frequency. Which one should we then choose in the short-run? Formal
learning theory broadens Reichenbach's general strategy by
considering what happens if we have other epistemic goals besides
long-run convergence to the truth. In particular, formal learning
theorists have considered the goal of getting to the truth as
efficiently, or quickly, as possible, as well as the goal of
minimising the number of mind-changes, or retractions along the way.
It has then been argued that the usual inductive method, which is
characterised by a preference for simpler hypotheses (Occam's
razor), can be justified since it is the unique method which meets the
standards for getting to the truth in the long run as efficiently as
possible, with a minimum number of retractions (Kelly 2007).



Steel (2010) has proposed that the Principle of Induction (understood
as a rule which makes inductive generalisations along the lines of the
Straight Rule) can be given a means-ends justification by showing that
following it is both necessary and sufficient for logical reliability.
The proof is an a priori mathematical one, thus it allegedly avoids
the circularity of Hume's second horn. However, Steel also does
not see the approach as an attempt to grasp Hume's first horn,
since the proof is only relative to a certain choice of epistemic
ends.



As with other results in formal learning theory, this solution is also
only valid relative to a given hypothesis space and conception of
possible sequences of data. For this reason, some have seen it as not
addressing Hume's problem of giving grounds for a particular
inductive inference (Howson 2011). An alternative attitude is that it
does solve a significant part of Hume's problem (Steel 2010).
There is a similar dispute over formal learning theory's
treatment of Goodman's riddle (Chart 2000, Schulte 2017).


### 5.5 Meta-induction



Another approach to pursuing a broadly Reichenbachian programme is
Gerhard Schurz's strategy based on meta-induction (Schurz 2008,
2017, 2019). Schurz draws a distinction between applying inductive
methods at the level of events--so-called
"object-level" induction (OI), and applying inductive
methods at the level of competing prediction methods--so-called
"meta-induction" (MI). Whereas object-level inductive
methods make predictions based on the events which have been observed
to occur, meta-inductive methods make predictions based on aggregating
the predictions of different available prediction methods according to
their success rates. Here, the success rate of a method is defined
according to some precise way of scoring success in making
predictions.



The starting point of the meta-inductive approach is that the aim of
inductive inference is not just, as Reichenbach had it, finding
long-run limiting frequencies, but also predicting successfully in
both the long and short run. Even if Hume has precluded showing that
the inductive method is reliable in achieving successful prediction,
perhaps it can still be shown that it is "predictively
optimal". A method is "predictively optimal" if it
succeeds best in making successful predictions out of all competing
methods, no matter what data is received. Schurz brings to bear
results from the regret-based learning framework in machine learning
that show that there is a meta-inductive strategy that is predictively
optimal among all predictive methods that are accessible to an
epistemic agent (Cesa-Bianchi and Lugosi 2006, Schurz 2008, 2017,
2019). This meta-inductive strategy, which Schurz calls
"wMI", predicts a weighted average of the predictions of
the accessible methods, where the weights are
"attractivities", which measure the difference between the
method's own success rate and the success rate of wMI.



The main result is that the wMI strategy is long-run optimal in the
sense that it converges to the maximum success rate of the accessible
prediction methods. Worst-case bounds for short-run performance can
also be derived. The optimality result forms the basis for an *a
priori* means-ends justification for the use of wMI. Namely, the
thought is, it is reasonable to use wMI, since it achieves the best
success rates possible in the long run out of the given methods.



Schurz also claims that this *a priori* justification of wMI,
together with the contingent fact that inductive methods have so far
been much more successful than non-inductive methods, gives rise to an
*a posteriori* non-circular justification of induction. Since
wMI will achieve in the long run the maximal success rate of the
available prediction methods, it is reasonable to use it. But as a
matter of fact, object-inductive prediction methods have been more
successful than non-inductive methods so far. Therefore Schurz says
"it is meta-inductively justified to favor object-inductivistic
strategies in the future" (Schurz 2019: 85). This justification,
he claims, is not circular because meta-induction has an *a
priori* independent justification. The idea is that since it is
*a priori* justified to use wMI, it is also *a priori*
justified to use the maximally successful method at the object level.
Since it turns out that that the maximally successful method is
object-induction, then we have a non-circular *a posteriori*
argument that it is reasonable to use object-induction.



Schurz's original theorems on the optimality of wMI apply to the
case where there are finitely many predictive methods. One point of
discussion is whether this amounts to an important limitation on its
claims to provide a full solution of the problem of induction. The
question then is whether it is necessary that the optimality results
be extended to an infinite, or perhaps an expanding pool of strategies
(Eckhardt 2010, Sterkenburg 2019, Schurz 2021a).



Another important issue concerns what it means for object-induction to
be "meta-inductively justified". The meta-inductive
strategy wMI and object-induction are clearly different strategies.
They could result in different predictions tomorrow, if OI would stop
working and another method would start to do better. In that case, wMI
would begin to favour the other method, and wMI would start to come
apart from OI. The optimality results provide a reason to follow wMI.
How exactly does object-induction inherit that justification? At most,
it seems that we get a justification for following OI on the next
time-step, on the grounds that OI's prediction approximately
coincides with that of wMI (Sterkenburg 2020, Sterkenburg
(forthcoming)). However, this requires a stronger empirical postulate
than simply the observation that OI has been more successful than
non-inductive methods. It also requires something like that "as
a matter of empirical fact, the strategy OI has been so much more
successful than its competitors, that the meta-inductivist attributes
it such a large share of the total weight that its prediction
(approximately) coincides with OI's prediction"
(Sterkenburg 2020: 538). Furthermore, even if we allow that the
empirical evidence does back up such a strong claim, the issue remains
that the meta-inductive justification is in support of following the
strategy of meta-induction, not in support of the *strategy* of
following OI (Sterkenburg (2020), sec. 3.3.2).


## 6. Living with Inductive Skepticism



So far we have considered the various ways in which we might attempt
to solve the problem of induction by resisting one or other premise of
Hume's argument. Some philosophers have however seen his
argument as unassailable, and have thus accepted that it does lead to
inductive skepticism, the conclusion that inductive inferences cannot
be justified. The challenge then is to find a way of living with such
a radical-seeming conclusion. We appear to rely on inductive inference
ubiquitously in daily life, and it is also generally thought that it
is at the very foundation of the scientific method. Can we go on with
all this, whilst still seriously thinking none of it is justified by
any rational argument?



One option here is to argue, as does Nicholas Maxwell, that the
problem of induction is posed in an overly restrictive context.
Maxwell argues that the problem does not arise if we adopt a different
conception of science than the 'standard empiricist' one,
which he denotes 'aim-oriented empiricism' (Maxwell
2017).



Another option here is to think that the significance of the problem
of induction is somehow restricted to a skeptical context. Hume
himself seems to have thought along these lines. For instance he
says:



> 
> 
> Nature will always maintain her rights, and prevail in the end over
> any abstract reasoning whatsoever. Though we should conclude, for
> instance, as in the foregoing section, that, in all reasonings from
> experience, there is a step taken by the mind, which is not supported
> by any argument or process of the understanding; there is no danger,
> that these reasonings, on which almost all knowledge depends, will
> ever be affected by such a discovery. (E. 5.1.2)
> 
> 
> 



Hume's purpose is clearly not to argue that we should not make
inductive inferences in everyday life, and indeed his whole method and
system of describing the mind in naturalistic terms depends on
inductive inferences through and through. The problem of induction
then must be seen as a problem that arises only at the level of
philosophical reflection.



Another way to mitigate the force of inductive skepticism is to
restrict its scope. Karl Popper, for instance, regarded the problem of
induction as insurmountable, but he argued that science is not in fact
based on inductive inferences at all (Popper 1935 [1959]). Rather he
presented a deductivist view of science, according to which it
proceeds by making bold conjectures, and then attempting to falsify
those conjectures. In the simplest version of this account, when a
hypothesis makes a prediction which is found to be false in an
experiment, the hypothesis is rejected as falsified. The logic of this
procedure is fully deductive. The hypothesis entails the prediction,
and the falsity of the prediction refutes the hypothesis by modus
tollens. Thus, Popper claimed that science was not based on the
extrapolative inferences considered by Hume. The consequence then is
that it is not so important, at least for science, if those inferences
would lack a rational foundation.



Popper's account appears to be incomplete in an important way.
There are always many hypotheses which have not yet been refuted by
the evidence, and these may contradict one another. According to the
strictly deductive framework, since none are yet falsified, they are
all on an equal footing. Yet, scientists will typically want to say
that one is better supported by the evidence than the others. We seem
to need more than just deductive reasoning to support practical
decision-making (Salmon 1981). Popper did indeed appeal to a notion of
one hypothesis being better or worse "corroborated" by the
evidence. But arguably, this took him away from a strictly deductive
view of science. It appears doubtful then that pure deductivism can
give an adequate account of scientific method.