## 1. What is Metaphysical Realism?



Metaphysical realism is the thesis that the objects, properties and
relations the world contains, collectively: the structure of the world
[Sider 2011], exists independently of our thoughts about it or our
perceptions of it. Anti-realists either doubt or deny the existence of
the structure the metaphysical realist believes in or else doubt or
deny its independence from our conceptions of it. Realists about
numbers, for example, hold that numbers exist mind-independently. This
view is opposed by Nominalists who deny the existence of abstract
objects and Intuitionists who agree numbers exist, but as mental
constructions, denying their mind-independence. Some realists about
laws of nature, to take an empirical example, hold that laws are
relations between universals [Armstrong 1983], others that laws are
ontologically primitive entities [Maudlin 2007]. Anti-realists about
laws of nature, on the other hand, either deny there are any laws at
all [Cartwright 1983; van Fraassen 1989] or else discern a dependence
on human concepts in the nature of these laws, interpreting them as
expressing certain expectations we have about regularities that we
unconsciously project onto the world [Blackburn 1986]. 



Metaphysical realism is not the same as scientific realism. That the
world's constituents exist mind-independently does not entail
that its constituents are as science portrays them. One could adopt an
instrumentalist attitude toward the theoretical entities posited by
science, continuing to believe that whatever entities the world
actually does contain exist independently of our conceptions and
perceptions of them. For the same reason, metaphysical realists need
not accept that the entities and structures ontologists posit exist
mind-independently.



Henceforth, we shall often just use the term 'realism' to
mean metaphysical realism. Opposition to realism can take many forms
so there is no single theoretical view denoted by the term
'anti-realism'. In particular, anti-realism is not
Idealism, even though Idealism is its most recognised form. One
approach, popular in Continental Philosophy, rejects realism on the
grounds that words can only acquire their meaning
*intra-linguistically*, through their semantic relations with
other words, where these relations are grounded in our linguistic and
cultural practices, rather than through referential relations to the
world outside of language. This view, Anti-Representationalism as it
is sometimes called, has gained traction in analytic philosophy also
[See Price 2009]. 



Within the ranks of Analytic Philosophy, Verificationists and
Pragmatists also reject realism, though for different reasons. We
shall mainly focus in this entry on the types of criticism voiced by
these two groups of Analytic philosophers with Michael Dummett
advocating a certain kind of Verificationism and Hilary Putnam a
certain kind of Pragmatism. While accepting Representationalism, both
Dummett and Putnam rejected realism by deploying semantic
considerations in arguments designed to show that realism is
untenable. The main goal of this entry is to outline these
'semantic' challenges to realism and to review realist
responses to them.



The characterization of realism in terms of mind-independence above is
not universally accepted. Some object that mind-independence is
obscure [e.g. Chalmers 2009; for relevant discussion, see the entry
 ontological dependence].
 Others maintain that realism is committed, in addition, to a
distinctive (and tendentious) conception of truth [Putnam 1981, 1985,
1992; Wright 1993; Button 2013; Taylor 2006] or, more radically, that
realism just is a thesis about the nature of truth--that truth
can transcend the possibility of verification, ruling statements for
which we can gather no evidence one way or the other to be
determinately either true or false. An example would be "Julius
Caesar's heart skipped a beat as he crossed the Rubicon."
Thus the realist on this view is one who believes the law of bivalence
(every statement is either true or false) holds for all meaningful
(non-vague) statements [Dummett 1978, 1991, 1993]. 



In the same vein, Crispin Wright [1992a, 2003] presents a nuanced
alethic analysis according to which discourses may be more or less
realist depending on which distinctive 'marks of truth'
they satisfy. 



These semantic formulations of metaphysical realism are unacceptable
to realists who are deflationists about truth, denying that truth is a
substantive notion that can be used to characterise alternative
metaphysical views [see the entry on the
 deflationary theory of truth].
 Realists collectively complain, with some justice, that the
anti-realist arguments are really arguments against the correspondence
(or other substantive) theory of truth rather than realism [Devitt
1983, 1991; Millikan 1986]. This is an important reason for preferring
an ontological construal of realism rather than a semantic one. 



There is, in fact, an obvious worry about using the notion of
mind-independence to characterise realism: it appears to consign
mental states and events to irreality. Surely your savouring the taste
of espresso, say, is dependent on your mind if anything is? Moreover,
just as certainly the nature and content of the experience of tasting
espresso depends upon one's beliefs and expectations. Indeed,
but this is not what the 'mind-independence'
characterisation of realism means to exclude. Rather, it is the
*existence* of conscious events that is deemed to be
independent of the particular opinions or theories we might hold about
their existence --given conscious events do exist, were our
descendants to uniformly dismiss them as illusory, they would be
mistaken. The 'mind-independence' at issue is epistemic
rather than ontological.



On this understanding of realism, it is an error to identify realism
with factualism, the view that sentences in some discourse or theory
are to be construed literally as fact-stating ones. The anti-realist
views discussed below are factualist about discourse describing
certain contentious domains. Adopting a non-factualist or
error-theoretic interpretation of some domain of discourse commits one
to anti-realism about its entities. Factualism is thus a necessary
condition for realism. But it is not sufficient. Verificationists such
as Dummett reject the idea that something might exist without our
being able to recognize its existence. They can be factualists about
entities such as numbers and quarks while anti-realists about their
nature since they deny any entities can exist mind-independently.



To elaborate the notion of mind-independent existence, consider Peter
van Inwagen's argument for the existence of numbers [van Inwagen
2016], which he describes as "a typical neo-Quinean
argument". The argument rests on two Quinean theses, van Inwagen
relays. Firstly, that there is only one kind of variable, a variable
that occupies nominal position, the range of which is unrestricted.
Secondly, that the meanings of the quantifiers are
 univocal.[1]



Given these two background assumptions, the argument he gives is this:



1. There are objects that have both mass and volume.
2. The average density of an object that has both mass and volume is
the ratio of its mass to its volume.
3. So, there is at least one thing that is a ratio.
4. Anything that is a ratio is a number.
5. Hence, there are numbers.



(3) follows from (1) and (2), with the conclusion (5) deducible from
(1), (2) and (4). Moreover, both (2) and (4) are, if not analytically
true, simple mathematical truths. (1), on the other hand, is an
empirical fact, van Inwagen notes. The argument is clearly valid and
the three premises that support the conclusion are all highly
plausible. Should we not just accept that numbers exist? Many
philosophers think so but some philosophers demur. Amongst the latter
are those who think that the meaning of 'there exists'
varies from context to context [Hilary Putnam and Eli Hirsch are two
prominent advocates whose ideas we review]. There are others who think
that the existential quantifier carries no ontological import
[Azzouni, J. 1997]. If one accepts Quine's two assumptions about
the existential quantifier, however, and regards the argument as
sound, hasn't one thereby accepted realism about numbers? 



No. For, while the argument establishes the existence of numbers, if
it is indeed sound, it leaves their nature unspecified. Hence, it does
not prove that numbers exist independently of human (or other) minds.
Moreover, since the inferences are intuitionistically valid,
anti-realists can accept it. The argument gives Intuitionists who
believe numbers are mental constructs just as much as Platonists who
believe they are eternal abstract objects a reason to believe numbers
exist. 


## 2. Mind-Independent Existence



Why do some find the notion of mind-independent existence inadequate
for the task of formulating metaphysical realism? The most common
complaint is that the notion is either obscure, or, more strongly,
incoherent or cognitively meaningless. An eloquent spokesman for this
strong view was Rudolf Carnap: "My friends and I have maintained
the following theses," Carnap announces [Carnap 1963,
p.868]:



> 
> (1) The statement asserting the reality of the external world
> (realism) as well as its negation in various forms, e.g. solipsism and
> several forms of idealism, in the traditional controversy are
> *pseudo-statements*, i.e., devoid of cognitive content. (2) The
> same holds for the statements about the reality or irreality of
> *other minds* (3) and for the statements of the reality or
> irreality of *abstract entities* (realism of universals or
> Platonism, vs. nominalism).
> 



In spite of his finding these disputes meaningless, Carnap indicates
how he thinks we could reconstruct them (sic.) so as to make some
sense of them: if we were to "replace the ontological theses
about the reality or irreality of certain entities, theses which we
regard as pseudo-theses, by proposals or decisions concerning the use
of certain languages. Thus realism is replaced by the practical
decision to use the reistic language".



Carnap does not have in mind a factualist reformulation of
metaphysical realism here--his "reistic" language is
strictly limited to the description of "intersubjectively
observable, spatio-temporally localized things or events".



What matters here is not Carnap's sense of a commensurability
between a metaphysical thesis about reality and a practical decision
to speak only about observable things, but rather that he thinks he
can explain how the illusion of meaningfulness arises for the
metaphysical theses he declares "devoid of cognitive
content". His explanation has to do with a distinction between
two types of questions -- *internal* and *external*
questions: 



> 
> An existential statement which asserts that there are entities of a
> specified kind can be formulated as a simple existential statement in
> a language containing variables for these entities. I have called
> existential statements of this kind, formulated *within* a
> given language, *internal* existential statements. [Carnap
> 1963, p. 871]
> 



Whereas internal questions about the existence of physical objects are
to be answered by observations that confirm or disconfirm sentences
asserting their existence, existential statements about abstract
objects are analytic, Carnap contends:



> 
> Just because internal statements are usually analytic and trivial, we
> may presume that the theses involved in the traditional philosophical
> controversies are not meant as internal statements, but rather as
> *external* existential statements; they purport to assert the
> existence of entities of the kind in question not merely within a
> given language, but, so to speak, before a language has been
> constructed. [1963, p. 871]
> 



Having dismissed all external existential questions as devoid of
cognitive content, Carnap decides that both realism which asserts the
ontological reality of abstract entities and nominalism which asserts
their irreality are "pseudo-statements if they claim to be
theoretical statements" (*ibid*).



Where Carnap could make no sense of the notion of mind-independent
reality, Albert Einstein had no such difficulty. For, together with
Podolsky and Rosen, Einstein famously proposed a test for elements of
reality in their EPR paper [Einstein, Podolsky and Rosen 1935:
777-8]:



> 
> *If, without in any way disturbing a system, we can predict with
> certainty (i.e. with probability equal to unity) the value of a
> physical quantity, then there exists an element of physical reality
> corresponding to this physical quantity*.
> 



Tim Maudlin [2014, p.7] explains the significance of the EPR criterion
thus:



> 
> Suppose ... I can *without in any way disturbing a system*
> predict with certainty the value of a physical quantity ... then the
> relevant element of reality obtained ... *independently of the
> determination being made.* Because, as we have said, the means of
> determination did not (by hypothesis) disturb the system.
> 



Realists might wish to endorse the EPR criterion as an idealized test
for the mind-independent reality of (macro-)physical quantities
-- even if we rarely (if at all) are able to "predict with
certainty" the outcome of an experiment, we can, some would
argue, approach near-certainty in a significant class of cases without
disturbing the system. Carnapian anti-realists, however, will deny the
EPR criterion validates any external notion of mind-independent
existence, regarding it instead as a means of settling internal
existence questions about physical quantities.


## 3. The Anti-Realist Challenges to Metaphysical Realism


### 3.1 Language Use and Understanding



The first anti-realist challenge to consider focuses on the use we
make of our words and sentences. The challenge is simply this: what
aspect of our linguistic use could provide the necessary evidence for
the realist's correlation between sentences and mind-independent
states of affairs? Which aspects of our semantic behaviour manifest
our grasp of these correlations, assuming they do hold?



For your representations of the world to be reliable, there must be a
correlation between these representations and the states of affairs
they portray. So the cosmologist who utters the statement "the
entropy of the Big Bang was remarkably low" has uttered a truth
if and only if the entropy of the Big Bang was remarkably low.



A natural question to ask is how the correlation between the statement
and the mind-independent state of affairs which makes it true is
supposed to be set up. One suggestive answer is that the link is
effected by the use speakers make of their words, the statements they
endorse and the statements they dissent from, the rationalizations
they provide for their actions and so forth; cognitively, it will be
the functional role of mental symbols in thought, perception and
language learning etc. that effects these links.



However, when we look at how speakers actually do use their sentences,
anti-realists argue, we see them responding not to states of affairs
that they cannot in general detect but rather to agreed upon
conditions for asserting these sentences. Scientists assert "the
entropy of the Big Bang was remarkably low" because they all
concur that the conditions justifying this assertion have been
met.



What prompts us to use our sentences in the way that we do are the
public justification conditions associated with those sentences,
justification conditions forged in linguistic practices which imbue
these sentences with meaning.



The realist believes we are able to mentally represent
mind-independent states of affairs. But what of cases where everything
that we know about the world leaves it unsettled whether the relevant
state of affairs obtains? Did Socrates sneeze in his sleep the night
before he took the hemlock or did he not? How could we possibly find
out? Yet realists hold that the sentence "Socrates sneezed in
his sleep the night before he took the hemlock" will be true if
Socrates did sneeze then and false if he did not and that this is a
significant semantic fact.



The Manifestation challenge to realism is to isolate some feature of
the use agents make of their words, or their mental symbols, which
forges the link between mind-independent states of affairs and the
thoughts and sentences that represent them. Nothing in the
thinker's linguistic behaviour, according to the anti-realist,
provides evidence that this link has been forged--linguistic use
is keyed to public assertibility conditions, not undetectable
truth-conditions. In those cases, such as the Socrates one, where we
cannot find out whether the truth-condition is satisfied or not, it is
simply gratuitous to believe that there is anything we can think or
say or do which could provide evidence that the link has been set up
in the first place. So the anti-realist claims [Dummett 1978, 1991,
1993; Tennant 1987, 1997; Wright 1993].



Why should we expect the evidence to be behavioural rather than, say,
neurophysiological? The reason anti-realists give is that the meanings
of our words and (derivatively for them) the contents of our thoughts
are essentially communicable and thus must be open for all speakers
and thinkers to see [Dummett 1978, 1993].


### 3.2 Language Acquisition



The second challenge to be considered concerns our acquisition of
language. The challenge to realism is to explain how a child could
come to know the meanings of certain sentences within his/her
language: the ones which the realist contends have undetectable
truth-makers associated with them. How could the child learn the
meanings of such sentences if these meanings are determined by states
of affairs not even competent speakers can detect?



Consider the sentence (S) once more:


* (S)  Socrates sneezed
in his sleep the night before he took the hemlock.



Realists say (S) is either true or false even though we may (and
almost certainly will) never know which it is. The state of affairs
which satisfies (S)'s truth-condition when it is true, its
'truthmaker', and the state of affairs which satisfies the
truth-condition of the negation of (S) when (S) is false are supposed
to be able to hold even though competent speakers cannot detect
whether they do. How could the child ever learn about this
undetectable relation?



Suppose God (or nature) had linked our mental representations to just
the right states of affairs in the way required by the realist. If so,
this is a semantically significant fact. Anyone learning their native
language would have to grasp these correspondences between sentences
and states of affairs. How can they do this if even the competent
speakers whom they seek to emulate cannot detect when these
correspondences hold? In short, competence in one's language
would be impossible to acquire if realism were true [Dummett 1978,
1993; Wright 1993]. This is the Language Acquisition challenge.



This challenge is exacerbated by the anti-realist's assumption
that since the linguistic meaning of an expression \(E\) is determined
solely by competent speakers' use of \(E\) the child's
task in all cases is to infer the meaning of \(E\) from its use. Thus
Dummett [1978 pp. 216-217], in discussing the meaning of
mathematical statements, proposes a thesis he argues holds for the
meanings of every kind of statement:



> 
> 
> The meaning of a mathematical statement determines and is exhaustively
> determined by its use. The meaning of a mathematical statement cannot
> be, or contain as an ingredient, anything which is not manifest in the
> use made of it, lying solely in the mind of the individual who
> apprehends that meaning: if two individual agree completely about the
> use to be made of the statement, then they agree about its meaning.
> The reason is that the meaning of a statement consists solely in its
> role as an instrument of communication between individuals, just as
> the powers of a chess-piece consist solely in its role in the game
> according to the rules.
> 
> 
> 



W.V.O. Quine is even more insistent on the public nature of linguistic
meaning. Displaying his unshakable faith in Skinnerian models of
language-learning he writes [1992, pp. 37-38]:



> 
> In psychology one may or may not be a behaviourist, but in linguistics
> one has no choice ... There is nothing in linguistic meaning
> beyond what is to be gleaned from overt behaviour in observable
> circumstances.
> 


### 3.3 Radical Skepticism



According to Hilary Putnam, the metaphysical realist subscribes not
just to the belief in a mind-independent world but also to the thesis
that truth consists in a correspondence relation between words (or
mental symbols) and things in that mind-independent world. Call this
thesis *correspondence truth* (after Devitt 1991). More
importantly, metaphysical realists aver that an ideal theory of the
world could be *radically* false, Putnam contends:
'radical' in the sense that **all** (or
almost all) of the theory's theses could fail to hold. Such a
global failure would result if we were to be
'brains-in-a-vat' our brains manipulated by mad scientists
(or machines, as in the movie *The Matrix*) so as to dream of
an external world that we mistake for reality. Call this thesis
*radical skepticism*.



It is widely believed that states of affairs that are truly
mind-independent do engender radical skepticism. The skeptic contends
that for all we could tell we could be brains in a vat--brains
kept alive in a bath of nutrients by mad alien scientists. All our
thoughts, all our experience, all that passed for science would be
systematically mistaken if we were. We'd have no bodies although
we thought we did, the world would contain no physical objects, yet it
would seem to us that it did, there'd be no Earth, no Sun, no
vast universe, only the brain's deluded representations of such.
At least this could be the case if our representations derived even
part of their content from links with mind-independent objects and
states of affairs. Since realism implies that such an absurd
possibility could hold without our being able to detect it, it has to
be rejected, according to anti-realists.



A much stronger anti-realist argument due to Putnam uses the
brain-in-a-vat hypothesis to show that realism is internally
incoherent rather than, as before, simply false. A crucial assumption
of the argument is semantic externalism, the thesis that the reference
of our words and mental symbols is partially determined by contingent
relations between thinkers and the world. This is a semantic
assumption many realists independently endorse.



Given semantic externalism, the argument proceeds by claiming that if
we were brains in a vat we could not possibly have the thought that we
were. For, if we were so envatted, we could not possibly mean by
'brain' and 'vat' what unenvatted folk mean by
these words since our words would be connected only to neural impulses
or images in our brains where the unenvatteds' words are
connected to real-life brains and real-life vats. Similarly, the
thought we pondered whenever we posed the question "am I a brain
in a vat?" could not possibly be the thought unenvatted folk
pose when they ask themselves the same-sounding question in English.
But realism entails that we could indeed be brains in a vat. As we
have just shown that were we to be so, we could not even entertain
this as a possibility, Putnam concludes that realism is incoherent
[Putnam 1981]. For this argument to work, however, Putnam must be
assuming a rather restrictive form of modal rationalism: we could be
brains in a vat only if in the circumstance that we were envatted, we
could conceive that we were envatted.


### 3.4 Models and Reality



Putnam's Model-Theoretic Argument is the most technical of the
arguments we have so far considered although we shall not reproduce
all the technicalities here. The central ideas can be conveyed
informally, although some technical concepts will be mentioned where
necessary. The argument purports to show that the Representation
Problem--to explain how our mental symbols and words get hooked
up to mind-independent objects and how our sentences and thoughts
target mind-independent states of affairs--is insoluble.



According to the Model-Theoretic Argument, there are simply too many
ways in which our mental symbols can be mapped onto items in the
world. The consequence of this is a dilemma for the realist. The first
horn of the dilemma is that s/he must accept that what our symbols
refer to is massively indeterminate. The second horn is that s/he must
insist that even an ideal theory, whose terms and predicates can
demonstrably be mapped veridically onto objects and properties in the
world might still be false, i.e., that such a mapping might not be the
right one, the one 'intended'.



Neither alternative can be defended, according to anti-realists.
Concerning the first alternative, massive indeterminacy for perfectly
determinate terms is absurd. As for the second, for realists to
contend that even an ideal theory could be false is to resort to
unmotivated dogmatism, since on their own admission we cannot tell
which mapping the world has set up for us. Such dogmatism leaves the
realist with no answer to a skepticism which undermines any capacity
to reliably represent the world, anti-realists maintain.



Now, in logic theories are treated as sets of sentences and the
objects (if any) that sentences talk about appear as elements of the
domain of set-theoretic entities called *structures*. Associated
with these structures are *interpretation functions* that map
individual constants onto individual objects of the domain and n-place
predicates onto n-tuples of elements in the domain. When a structure
makes all the sentences of a given theory true it is called a
*model* of the theory. By demonstrating that there is a model of
T we show theory T is consistent. If T turns out to be true in its
*intended model*, then T is true *simpliciter*. 



> 
> 
> For an informal illustration of the basic ideas of model theory, see
> the supplementary document,
>  Model Theory: Core Ideas.
>  
> 
> 
> 



Let us call structures whose domains consist of numbers
'numeric' structures. The nub of Putnam's
Model-Theoretic Argument against realism is that the realist cannot
distinguish the intended model for his/her total theory of the world
from non-standard interlopers such as permuted models or ones derived
from numeric models, even when total theory is a rationally optimal
one that consists, as it must do, of an *infinite* set of
sentences and the realist is permitted to impose the most exacting
constraints to distinguish between models. This is a very surprising
result if true! How does Putnam arrive at it? 



Putnam actually uses a number of different arguments to establish the
conclusion above. Of prime concern to realists, as Taylor (2006)
emphasises, is the argument based on Godel's Completeness
Theorem, GCT. For, following Lewis [Lewis, 1984], realists might
concede to Putnam that they cannot single out the intended model or
distinguish it from various ersatz models, but argue that this is not
necessary since it suffices that an intended model *exists*,
even if we cannot specify it. This response does not answer the GCT
argument, however. For this argument purports to prove directly that
an ideal theory of the world could not be false, a conclusion flatly
inconsistent with realism. See the supplementary document
 The Model-Theoretic Argument and the Completeness Theorem
 for an outline of this argument. 



Putnam has another argument, the Permutation Argument:



Suppose that the realist is able to somehow specify the intended
model. Call this intended model \(W''\). Then nothing the realist can
do can possibly distinguish \(W''\) from a permuted variant \(W^\*\)
which can be specified following Putnam 1994b, 356-357:



> 
> We define properties of being a cat\* and being a mat\* such that:
> 
> 1. In the actual world cherries are cats\* and trees are mats\*.
> 2. In every possible world the two sentences "A cat is on a
> mat" and "A cat\* is on a mat\*" have precisely the
> same truth value.
> 
> 
> 



Instead of considering two sentences "A cat is on a mat"
and "A cat\* is on a mat\*" now consider only the one
"A cat is on a mat", allowing its interpretation to change
by first adopting the standard interpretation for it and then adopting
the non-standard interpretation in which the set of cats\* are assigned
to 'cat' in every possible world and the set of mats\* are
assigned to 'mat' in every possible world. The result will
be the truth-value of "A cat is on a mat" will not change
and will be exactly the same as before in every possible world.
Similar non-standard reference assignments could be constructed for
all the predicates of a language. [See Putnam 1985, 1994b.] However,
unlike the GCT argument, the Permutation Argument is susceptible to
the Lewis-styled reply above.


### 3.5 Conceptual Schemes and Pluralism



According to conceptual pluralists, there can no more be an answer to
the question "What is the structure of the world?" outside
of some scheme for classifying entities than there can be an answer to
the question of whether two events \(A\) and \(B\) are simultaneous
outside of some inertial frame for dating those events. The objects
that exist are the objects some conceptual scheme says
exists--'mesons exist' really means 'mesons
exist relative to the conceptual scheme of current physics'.



Realists think there is a unitary sense of 'object',
'property' etc., for which the question "what
objects and properties does the world contain?" makes sense. Any
answer which succeeded in listing all the objects, properties, events
etc. which the world contains would comprise a privileged description
of that totality [Putnam 1981]. Anti-realists reject this. For them
'object', 'property' etc., shift their senses
as we move from one conceptual scheme to another. Some anti-realists
argue that there cannot be a totality of all the objects the world
contains since the notion of 'object' is indefinitely
extensible and so, trivially, there cannot be a privileged description
of any such totality. [For discussions of indefinite extensibility see
Dummett 1978, 1991; Linnebo 2018; Warren 2017].



How does the anti-realist defend conceptual relativity? One way is by
arguing that there can be two complete theories of the world which are
descriptively equivalent yet logically incompatible from the
realist's point of view. For example, theories of space-time can
be formulated in one of two mathematically equivalent ways: as an
ontology of points, with spatiotemporal regions being defined as sets
of points; or as an ontology of regions, with points being defined as
convergent sets of regions. Such theories are descriptively equivalent
since mathematically equivalent and yet are logically incompatible
from the realist's point of view, anti-realists contend [Putnam
1985, 1990].


## 4. Realist Responses


### 4.1 Language Use and Understanding



We now turn to some realist responses to these challenges. The
Manifestation and Language Acquisition arguments allege there is
nothing in an agent's cognitive or linguistic behaviour that
could provide evidence that s/he had grasped what it is for a sentence
to be true in the realist's sense of 'true'. How can
you manifest a grasp of a notion which can apply or fail to apply
without you being able to tell which? How could you ever learn to use
such a concept?



One possible realist response is that the concept of truth is actually
very simple, and it is spurious to demand that one always be able to
determine whether a concept applies. As to the first part, it is often
argued that all there is to the notion of truth is what is given by
the formula "'\(p\)' is true if and only if
\(p\)". The function of the truth-predicate is to disquote
sentences in the sense of undoing the effects of quotation--thus
all that one is saying in calling the sentence "Yeti are
vicious" *true* is that Yeti are vicious.



It is not clear that this response really addresses the
anti-realist's worry, however. It may well be that there is a
simple algorithm for learning the meaning of 'true' and
that, consequently, there is no special difficulty in learning to
apply the concept. But that by itself does not tell us whether the
predicate 'true' applies to cases where we cannot
ascertain that it does. All the algorithm tells us, in effect, is that
if it is legitimate to assert \(p\) it is legitimate to assert that
'\(p\)' is true. So are we entitled to assert
'either Socrates did or did not sneeze in his sleep the night
before he took the hemlock' or are we not? Presumably that will
depend on what we mean by the sentence, whether we mean to be
adverting to two states of affairs neither of which we have any
prospect of ever confirming.



Anti-realists follow verificationists in rejecting the intelligibility
of such states of affairs and tend to base their rules for assertion
on intuitionistic logic, which rejects the universal applicability of
the Law of Bivalence (the principle that every statement is either
true or false). This law is thought to be a foundational semantic
principle for classical logic. However, some question whether
classical logic requires bivalence [e.g. Sandqvist 2009]. Others
dispute the idea that acceptance or rejection of bivalence has any
*metaphysical* (rather than meaning-theoretic) consequences
[Edgington, 1981; McDowell 1976; Pagin 1998; Gaiffman 1996]. There is,
in addition, a question as to whether the anti-realist's
preferred substitute for realist truth-conditions in
verification-conditions (or proof-conditions) satisfies the
requirement of exhaustive manifestability [Pagin 2009]. 



A more direct realist response to the Manifestation challenge points
to the prevalence in our linguistic practices of realist-inspired
beliefs to which we give expression in what we say and do [McDowell
1976]. We assert things like "either there were an odd or an
even number of dinosaurs on this planet independently of what anyone
believes" and all our actions and other assertions confirm that
we really do believe this. Furthermore, the overwhelming acceptance of
classical logic by mathematicians and scientists and their rejection
of intuitionistic logic for the purposes of mainstream science
provides very good evidence for the coherence and usefulness of a
realist understanding of truth [Edgington 1981; Burgess 1984; Hellman
1989, 1992]. 



Anti-realists reject this reply. They argue that all we make manifest
by asserting things like "either there were an odd or an even
number of dinosaurs on this planet independently of what anyone
believes" is our pervasive misunderstanding of the notion of
truth. They apply the same diagnosis to the realist's belief in
the mind-independence of entities in the world and to counterfactuals
which express this belief. We overgeneralize the notion of truth,
believing that it applies in cases where it does not, they contend
[Tennant 1987, 1997; Wright 1993]. 



An apparent consequence of their view is that reality is indeterminate
in surprising ways--we have no grounds for asserting that
Socrates did sneeze in his sleep the night before he took the hemlock
and no grounds for asserting that he did not and no prospect of ever
finding out which. Does this mean that for anti-realists the world
contains no such fact as the fact that Socrates did one or the other
of these two things? Not necessarily. For anti-realists who subscribe
to intuitionistic principles of reasoning, the most that can be said
is that there is no present warrant to assert \(S \lor \neg S\): that
Socrates either did or did not sneeze in his sleep the night before he
took the hemlock.



Perhaps anti-realists are right. But if so, they need to explain how a
practice based on a pervasive illusion can be as successful as modern
science. Anti-realists perturbed by the manifestability of realist
truth are revisionists about parts of our linguistic practice, and the
consequence of this revisionist stance is that mathematics and science
require extensive and non-trivial revision. The debate about whether
classical logic should (or can) be rejected on meaning-theoretic
grounds is ongoing: Burgess 1984, Hellman 1989, Michael 1999 and Read
2000 are critical of Dummett's case for rejecting classical
logic, whereas Cogburn 2005, Cozzo 1994, Prawitz 1977, 1987, 1994 and
Tennant 1997 are, in varying degrees, supportive. 


### 4.2 Language Acquisition



The challenge to realism posed by language acquisition is to explain
how a child (or novice) could come to know the meanings of certain
sentences within his/her language: the ones the realist contends have
undetectable truth-makers associated with them. How could the child
learn the meanings of such sentences if these meanings are determined
by states of affairs not even competent speakers can detect?



A straightforward realist reply is that knowledge of the meanings of
sentences with undetectable truth-conditions is acquired in the same
way as knowledge of the meanings of sentences with recognisable
truth-conditions: namely, compositionally by acquiring knowledge of
the lexicon and the relevant compositional principles [Pagin
 2009].[2]
 Anti-realists respond by contesting the interpretation of the
compositional principles that generate sentences with undetectable
truth-conditions -- where realists assert \(S \lor \neg S\) is
true (\(S\) being the Socrates sentence), anti-realists maintain there
is no ground for asserting this disjunction [e.g. Tennant 1987].



Some realists reject the publicity of meaning principle as it applies
to language learning. While many accept that the meaning of a word is
determined by its use in a given language, not all do [e.g. Chomsky
1986; Fodor and Lepore 2002]. Realists who think of semantic
understanding as a mental state reject the idea that a speaker's
understanding of meaning is exhaustively manifest in its use *as an
instrument of communication* rather than in its internal use in
expressing thought. If Dummett's manifestation requirement is a
demand for a behaviouristic reduction of semantic knowledge, they
argue, it should be rejected [Burgess 1984; Chomsky 1986]. However,
some sympathetic to the demand for full manifestability of semantic
knowledge reject the behaviouristic construal and instead justify it
on conceptual grounds [e.g. Shieh 1998, McGee
 2015].[3]




The Acquisition Challenge is a vexed one for realists because Dummett
shares little common ground with the many (realist) philosophers,
linguists and cognitive scientists who believe language acquisition is
effected by a dedicated language module [Fodor 1975, 1983, 2008;
Chomsky 1986, 2006; Crain 2012; Pinker 1994] or even with those who
disavow modularity but agree that semantic knowledge is partly
unconscious. Thus, Dummett rejected Chomsky's thesis that
speakers have unconscious knowledge of the rules of Universal Grammar
on the Wittgensteinian grounds that it was, at best "an
explanatory hypothesis, not a systematisation of facts open to
view." [Dummett 1981]. Dummett apparently took this
"systemisation of facts" to be satisfied by an account
that pairs knowledge of meaning with recognitional
 abilities.[4]



It is worth noting that evidence from developmental psychology
indicates some meaning is pre-linguistic and that some pre-linguistic
meaning or conceptual content relate to situations that are not
detectable by the child. For example, psychologists have discovered
systems of core knowledge activated in infancy that govern the
representation of, *inter alia* concrete objects and human
agents [see Spelke 2003; Spelke and Kinzler 2007]. An interesting
finding from preferential gaze experiments suggests 4 month old
infants represent occluded objects as continuing behind their
 barriers.[5]



While these findings do not by themselves show that the meanings of
mental symbols is not determined by public use, they do provide
evidence that 'verification-transcendent' conceptual
content is laid down in the earliest stages of cognitive development.



### 4.3 Radical Skepticism



The Brains-in-a-Vat argument purports to show that, given semantic
externalism, realism is incoherent on the grounds that it is both
committed to the genuine possibility of our being brains in a vat and
yet entails something that anti-realists judge to be inconsistent with
this: namely, that were we to be so envatted we could not possibly
have the thought that we were.



Realists have three obvious responses.


1. Deny realism entails that we could be brains in a vat.
2. Deny semantic externalism.
3. Deny there is any inconsistency between our being brains in a vat
and our inability to think that we were brains in a vat were we to be
so.



As for (i), one might question the coherence of the idea of our being
brains in a vat on the grounds that the skeptical hypothesis uses
terms which derive their meaning from successful theory to pose a
problem which, if intelligible, would rob those very terms of meaning.




What of option (ii)--denying semantic externalism? Is this really
a live prospect for realists? Semantic externalism no longer commands
the consensus amongst realists that it did when Putnam formulated his
Brains-in-a-Vat argument. David Lewis, a prominent realist, rejected
externalism in favour of a sophisticated semantic internalism based on
a 'Two-Dimensional' analysis of modality proposed by
Stalnaker [Lewis 1994]. Frank Jackson [Jackson 2000] contributed to
the development of internalist 2D semantics and used it to formulate a
version of materialism grounded on conceptual analysis that provides a
useful model of a physicalistic realist's metaphysics.



Other realists reject externalism because they think that the
Representation Problem is just a pseudo-problem. When we say things
like "'cat' refers to cats" or
"'quark' refers to quarks" we are simply
registering our dispositions to call everything we consider
sufficiently cat-like/quark-like,
'cat'/'quark' [Horwich, 1990; Resnick,
1990].



According to these semantic deflationists, it is just a confusion to
ask how the link was set up between our use of the term 'the Big
Bang' and the event of that name which occurred some fourteen
billion years ago. Yet, if all there is to the story are our
linguistic dispositions and the conditions to which they are presently
attuned, the case has effectively been ceded to the anti-realist who
denies it is possible to set up a correlation between our utterances
or thoughts and mind-independent states of affairs. 



Perhaps the most effective realist rejoinder is (iii). We shall return
to this response after we have reviewed Putnam's Brains-in-a-Vat
Argument, BIVA.



How does Putnam prove we can know we are not brains in a vat? To
understand Putnam's argument, we need to first recall the
'Twin-Earth' considerations used to support Semantic
Externalism: on Twin-Earth things are exactly as they are here on
Earth except for one difference--whereas for Earthly humans water
has the chemical composition H2O, for our
doppelgangers on Twin-Earth, twumans, water is instead composed
of some substance unknown to us on Earth, XYZ. Now when you and your
twuman counterpart say (or think) "'Water' refers to
water" both of you utter (or think) truths. But which truth you
both think or utter differs. For humans "'Water'
refers to water" expresses the truth that the term
'water' in English refers to that substance whose chemical
composition is H2O. For our twuman Twin-Earth counterparts,
however, their sentence "'Water' refers to
water" expresses the truth that their term 'water'
in Twenglish refers to that substance whose chemical composition is
XYZ.



With these points about Externalism in mind, consider Putnam's
BIVA [we follow the formulation section 7 of the entry
 skepticism and content externalism].
 Let us call whatever it is that an envatted brain's symbol
'tree' refers to, if it refers at all,
\(v\)-*trees*. Then the BIVA is: 


* (1)  If I'm a BIV
then it is not the case that if my word 'tree' refers it
refers to trees.
* (2)  If my word
'tree' refers it refers to trees.
* (3)  So, I am
*not* a BIV.



Now (1) seems correct: if I am a brain-in-a-vat then, given
externalism, my symbol 'tree' cannot refer to trees since
there aren't any trees in the vat-world--a BIV's
'tree' symbol refers to \(v\)-*trees*, *not*
trees. But what reason do we have to believe (2)? If we are BIVs
won't our 'tree' symbols refer to v-trees rather
than trees? Crispin Wright [1992b] argues that all language-users,
whether humans or brains-in-a-vat, can be certain of (2) since they
can know they use language meaningfully and thus can know that their
language disquotes. Graeme Forbes [1995] questions Wright's
argument. 



Discussion of the brains-in-a-vat hypothesis has been extensive. Early
contributions by Brueckner 1986, 1992; David 1991; Ebbs 1992; Forbes
1995 reconstruct Putnam's argument and assess it from a realist
perspective. Important defences of the BIVA are provided by Wright
1992b; Tymoczko 1989; Button 2013, 2015. Some recent discussions bring
Bayesian [Huemer 2016] or psychological [Jackson 2015] considerations
to bear on Putnam's BIV hypothesis. A valuable collection of
essays is Goldberg 2015.



Even if it were to turn out that the BIVA is not sound, Putnam's
challenge to the realist remains unanswered. This was to show how
realism could be coherent if it is committed both to:


* (I)  The real
possibility that we are brains-in-a-vat



and to the consequence that:


* (II)  Were we to be
BIVs we could not have the thought that we were.



While it is usually not remarked upon, there is no *logical*
incoherence in accepting both (I) and (II)--as the figure below
illustrates. There is thus no logical incoherence in believing both
that it is possible that one is a BIV and that if one is a BIV one
could never come to know this. 



![Four worlds: w* with not-v, Ev; and w', w'', w''' all with v and not-Ev. Each world is accessible from itself and w' is accessible from w* but only w'' and w''' are accessible from w'.](figure.png)

Figure. Consider a universe of 4 worlds.
If we are **not** in fact brains-in-a-vat (so that the hypothesis
\(v\) that we are brains in a vat is false, \(\neg v\) is true at
\(w^\*\), the actual world) we can nonetheless entertain \((E)\), the
hypothesis that we are (so that \(Ev\) is true at \(w^\*\)),
recognizing as we do so that were we to inhabit a world such as \(w'\)
in which we are brains-in-a-vat (\(v\) holds at \(w'\)), we would lack
the semantic resources to articulate thoughts reflecting our own
envatted state so that we *could not* so much as entertain the
thought that we were brains-in-a-vat (\(\neg Ev\) holds not only at
\(w'\) but at all those worlds ( \(w''\), \(w'''\)) accessible from
\(w'\)).




Nick Bostrom has recently argued it is quite likely that we humans are
actually *virtual humans*: computer simulations of flesh and
blood creatures. Bostrom reasons that if our mental lives can be
simulated then it is highly probable that our distant descendants
(more intelligent or at least more technologically advanced
'post-human' successors) will eventually create such a
simulation in which case it is more likely that we are the unwitting
denizens of a simulated world than the flesh and blood inhabitants of
the real world we take ourselves to be. At least this will be so
unless the chances that creatures of our intelligence are doomed to
become extinct before reaching the technological sophistication to
create simulations are overwhelmingly large or else almost no such
technologically capable civilizations have any interest in simulating
minds like ours in the first place [Bostrom, N., 2003]. 



Bostrom's position owes nothing to skepticism, he is concerned
solely with the question of whether virtual humans are empirically
possible and, if so, how likely it is that we might be such beings.
His argument, if sound, makes it look very doubtful that we can know
*a priori* that we are **not** brains-in-a-vat,
when BIVs are understood to be virtual humans in a
 simulation.[6]
 If Bostrom is correct, Putnam's attempt to prove we *cannot
be* BIVs must be flawed. However, the Simulation Argument is
nothing if not controversial: it has provoked interest from
cosmologists as well as philosophers [For discussion of the Simulation
Hypothesis see Bostrom, 2005; Brueckner 2008; Chalmers 2010;
Weatherson 2003]. 


### 4.4 Models and Reality



If metaphysical realism is to be tenable, it must be possible for even
the best theories to be mistaken. Or so metaphysical realists have
thought. Whence, such realists reject the Model-Theoretic Argument MTA
which purports to show that this is not possible. Here is an informal
sketch of the MTA due to van Fraassen [1997]:



> 
> Let \(T\) be a theory that contains all the sentences we insist are
> true, and that has all other qualities we desire in an ideal theory.
> Suppose moreover that there are infinitely many things, and that \(T\)
> says so. Then there exist functions (interpretations) which assign to
> each term in \(T\)'s vocabulary an extension, and which satisfy
> \(T\). So we conclude, to quote Putnam, "\(T\) comes out true,
> true of the world, provided we just interpret 'true' as
> TRUE(SAT)".
> 



Here 'TRUE(SAT)' means "true relative to a mapping
of the terms of the language of \(T\) onto (sets of) items in the
world".



But why should we interpret 'true' as TRUE(SAT)? Because
truth is truth in an intended model and, Putnam argues, amongst all
the models of \(T\) that make all its theses come out true there is
guaranteed to be at least one that passes all conceivable constraints
we can reasonably impose on a model in order for it to be an intended
model of \(T\). 



Realists have responded to the argument by rejecting the claim that a
model \(M\) of the hypothetical ideal theory \(T\) passes every
theoretical constraint simply because all of the theory's theses
come out true in it. For there is no guarantee, they claim, that terms
stand in the right relation of reference to the objects to which \(M\)
links them. To be sure, if we impose another theoretical constraint,
say:



> 
> Right Reference Constraint (RRC): Term \(t\) refers to object \(x\) if
> and only if \(Rtx\) where \(R\) is the right relation of reference,
> 



then \(M\) (or some model based on it) can interpret this RRC
constraint in such a way as to make it come out true.



But there is a difference between a model's making some
description of a constraint come out true and its actually conforming
to that constraint, metaphysical realists insist [Devitt 1983, 1991;
Lewis 1983, 1984].



For their part, anti-realists have taken the metaphysical
realist's insistence on a Right Reference Constraint to be
'just more theory'--what it is for a model to conform
to a constraint is for us to be justified in asserting that it does.
Unfortunately, this has led to something of a stand-off. Metaphysical
realists think that anti-realists are refusing to acknowledge a clear
and important distinction. Anti-realists think realists are simply
falling back on dogmatism at a crucial point in the argument.



On the face of it, the Permutation Argument presents a genuine
challenge to any realist who believes in determinate reference. But it
does not refute metaphysical realism unless such realism is committed
to determinate reference in the first place and it is not at all
obvious that this is so.



Realist responses to this argument vary widely. At one extreme are the
'determinatists', those who believe that Nature has set up
significant, determinate referential connections between our mental
symbols and items in the world. They contend that all the argument
shows is that the distribution of truth-values across possible worlds
is not sufficient to determine reference [van Cleve 1992].



At another extreme are 'indeterminatists', realists who
concede the conclusion, agreeing that it demonstrates that word-world
reference is massively indeterminate or 'inscrutable'. The
locus classicus for inscrutability of reference is Quine 1964 [See
also Quine 1969, 1992; Davidson 1979]. 



Some infer from this that reference could not possibly consist in
correspondences between mental symbols and objects in the world. For
them all that makes 'elephant' refer to elephants is that
our language contains the word 'elephant'. This is
Deflationism about reference. Vann McGee presents a strong case for
inscrutability on a deflationary view of reference, one that is
grounded in a "... peculiarly egocentric conception of
semantics--questions of others' meanings are settled by
asking what I mean by the words of my language" [McGee
2015].



In between these two extremes are those prepared to concede the
argument establishes the real possibility of a significant and
surprising indeterminacy in the reference of our mental symbols but
who take it to be an open question whether other constraints can be
found which pare down the range of reference assignments to just the
intuitively acceptable
 ones.[7]




The simplest and most direct response to the MTA questions its
validity. Thus Devitt and Lewis claim that Putnam's alternative
model \(M\) has not been shown to satisfy *every* theoretical
constraint *merely* by making some description of each
theoretical constraint true.



Skolem's Paradox in set theory seems to present a striking
illustration of Lewis's distinction. The Lowenheim-Skolem
Theorem states that every consistent, countable set of first-order
formulae has a denumerable model, in fact a model in the set of
integers \(\mathbb{Z}\). Now in ZF one can prove the existence of sets
with a non-denumerable number of elements such as the set
\(\mathbb{R}\) of real numbers. Yet the ZF axioms comprise a
consistent, countable set of first-order formulae and thus by the
Lowenheim-Skolem Theorem has a model in \(\mathbb{Z}\). So
ZF's theorem \(\phi\) stating that \(\mathbb{R}\) is
non-denumerable will come out true in a denumerable model \(\mu\) of
ZF.



How can this be? One explanation is that \(\mu\) makes \(\phi\) true
only at the cost of re-interpreting the term
'non-denumerable' so that it no longer means
*non-denumerable*. Thus \(\mu\) is not the intended model
\(M^\*\) of ZF. It looks as if the metaphysical realist has a clear
illustration of Lewis's distinction at hand in set theory.



Unfortunately for the realist, this is not the only explanation. In
fact, Putnam used this very example in an early formulation of the
MTA. Just because there are different models that satisfy \(\phi\) in
some of which \(\mathbb{R}\) is non-denumerable but in others of which
(such as \(\mu\)) \(\mathbb{R}\) is denumerable, Putnam argued, it is
impossible to pin down the intended interpretation of
'set' via first-order axioms. Moreover, well before
Putnam, Skolem and his followers had taken the moral of Skolem's
Paradox to be that set-theoretic notions are indeterminate [For
further discussion, see the entry on
 Skolem's paradox].



The question of how to interpret Skolem's Paradox merely raises
anew the question of what it is for a theory such as the hypothesized
ideal theory \(T\) to satisfy a right reference constraint, (RRC).



Putnam [1985] regards it as simply question-begging for a realist to
assume her notion of an intended model is determinate: i.e. that terms
such as 'satisfaction' or 'correspondence'
refer to those relations to which she wishes them to refer. That her
term 'refers' refers to her desired reference relation is
'just more theory'. Realists have responded that Putnam is
wilfully re-interpreting their semantic terms as he sees
 fit.[8]




Is there some independent way to validate Lewis's distinction?
Michael Resnick thinks so [Resnick 1987]. Putnam maintained that
\(M\), the model he constructs of the ideal theory \(T\), is an
intended model because it passes every operational and theoretical
constraint we could reasonably impose. It passes every theoretical
constraint, he argues, simply because it makes every thesis of \(T\)
true. But unless the Reflection Principle **(RP)** below
holds, Resnick argues, this inference is just a
*non-sequitur*:


* (**RP**) 
To any condition \(f\) that a model of a theory satisfies, there
corresponds a condition \(C\) expressible in the theory that that
theory satisfies.



However, this principle is false. The simplest counterexample to it,
Resnick points out, is Tarskian truth. Suppose we impose on
\(T\)'s model \(M\) a condition \(f^\*\) that \(M\) makes all of
\(T\)'s theses come out true. Then, unless \(T\) is either
inconsistent or too weak to express elementary arithmetic no truth
predicate will be definable in \(T\). Whence there will be no
condition \(C\) expressible in \(T\) corresponding to this condition
\(f^\*\) on \(T\)'s model(s) \(M\).



Resnick concludes (*ibid*):



> 
> Any true interpretation of \(T\) whatsoever--even one which does
> not satisfy \(C\)--will make true every thesis of \(T\),
> including T's assertion that \(C\) is satisfied. Which suffices
> to block the 'just more theory' gambit.
> 



The philosophical consensus appears to be that Lewis and Resnick are
right. Apart from the authors already discussed, important criticisms
of the MTA were advanced in Hale and Wright 1997, van Cleve 1992 and
Bays 2008. However, some very sophisticated anti-realist attempts to
buttress the Model-Theoretic Argument against Lewis-styled criticisms
have appeared. Igor Douven reconstructs Putnam's argument,
defending it against standard objections [Douven 1999]. Barry Taylor
presents a detailed explication and defence of Putnam's Just
More Theory reply [Taylor 2006], as does Tim Button [Button 2013].
Whether these newer formulations of the MTA succeed in answering the
Lewis/Resnick objection is an open
 question.[9]



### 4.5 Conceptual Schemes and Pluralism



To the extent it makes the existence of all things relative to the
classificatory skills of minds, conceptual relativism appears highly
counter-intuitive to realists. Whilst it may seem plausible to some
that moral values or perhaps even colours might disappear with the
extinction of sentient life on Earth, it is not at all plausible to
think that trees, rocks and microbes would follow in their train. 



This is not how Putnam understands his idea of conceptual relativity,
however, which thesis he distinguishes from conceptual relativism. As
he sees things, we accept a theory which licenses us to assert
"Electrons exist" and also licenses us to assert "if
humans were to disappear from this planet, electrons need not follow
in their train" since the theory assures us that the existence
of electrons in no way causally depends on the existence of humans.
For the anti-realist our well-founded practices of assertion ground at
one and the same time our conception of the world and our conception
of humanity's place within it.



Realists might still worry that whether there are to be any electrons
in the anti-realist's ontology apparently depends upon the
conceptual schemes humans happen to chance upon and Putnam himself
encourages this interpretation: "'objects' do not
exist independently of conceptual schemes" [Putnam 1981]. The
relativity of existence to conceptual scheme is, in this respect,
quite unlike the relativity of simultaneity to frame of reference.



Still, anti-realists maintain that there are actual instances of
conceptual schemes that explain the same phenomena equally well,
schemes which, they aver, realists must judge to be logically
incompatible. The earlier example of competing theories of space-time
was a case in point. On one theory, \(T\_1\), space-time consists of
unextended spatiotemporal points and regions of space-time are sets of
these points. According to the second theory, \(T\_2\), space-time
consists of extended spatiotemporal regions and points are logical
constructions--convergent sets of regions. Realists will judge
that only one of the two theories can be true if they really are
logically incompatible. Anti-realists respond that the two theories
\(T\_1\) and \(T\_2\) cannot differ in truth-value since they are
descriptively equivalent. 



Anti-realists regard two theories as descriptively equivalent if each
theory can be interpreted in the other and both theories explain the
same phenomena. Is there nothing more to the notion of descriptive
equivalence than this? Realists might not accept that there
isn't.



Consider our two competing theories of space-time \(T\_1\) and \(T\_2\)
again. Are \(T\_1\) and \(T\_2\) descriptively equivalent? At the stroke
of midnight Cinderella's carriage changes into a
pumpkin--it is a carriage up to midnight, a pumpkin thereafter.
According to the region-based theory \(T\_2\) which takes temporal
intervals as its primitives, that's all there is to it. But if
there are temporal points, instants, as \(T\_1\) affirms, there is a
further issue left undecided by this story--viz, at the moment of
midnight is the carriage still a carriage or is it a pumpkin?



So does the region-based theory fail to recognize certain facts or are
these putative facts merely artefacts of the punctate theory's
descriptive resources, reflecting nothing in reality? We cannot
declare the two theories \(T\_1\) and \(T\_2\) descriptively equivalent
until we resolve this question at least.



In fact, there is no reason why realists cannot agree with
anti-realists in regarding the conflict between a punctate geometry
and a region-based geometry as *merely* apparent. Thomas
William Barrett and Hans Halvorson argue that the two theories "
... are simply convenient ways of expressing the geometric facts that
are more fully expressed by a comprehensive theory" that
quantifies over both points and lines. \(T\_1\) and \(T\_2\) cannot be
incompatible according to Barrett and Halvorson because they are in
essence the *same* theory [Barrett and Halvorson 2017].
However, the geometric case is a rather special one.



Consider another Putnam-styled case [Putnam 2004]. Ernie looks into
his bag and sees there are 3 coins and nothing else and announces
"There are exactly 3 objects in my bag." Maxi looks into
Ernie's bag and shakes her head "No Ernie there are 7
objects in your bag!" she corrects him. The Carnapian pluralist
feels she can defuse the conflict and accommodate both points of view
by maintaining that whilst 3 objects exist-in-\(E\) (where \(E\) is
Ernie's everyday framework), 7 objects exist-in-\(M\) (with
\(M\) Maxi's mereological framework). But even if Maxi can
endorse both of these claims (since the mereological objects include
Ernie's 3 coins), it is not at all certain Ernie can do so. If
Ernie is unpersuaded that mereological fusions of objects are
themselves objects, then Maxi's putative truthmaker for her
framework-relative existence claim "7 objects
exist-in-\(M\)" will be unconvincing to him.



For this case, the pluralist's suggestion that 3 objects
exist-in-\(E\) but 7 objects exist-in-\(M\) is not clearly warranted.
There are simpler explanations: one is that by 'object'
Ernie means ordinary object, by 'object' Maxi means
mereological object. Nothing deeper than that is required to explain
their disagreement. Rather than existence or truth that is
relativized, the meanings of their terms differ. On this account,
pluralists have mistaken a plurality of meanings for a plurality of
modes of being. However, other explanations are also possible: for
instance, it may be that Ernie and Maxi do mean the same thing by
'object' but hold incompatible theories about what counts
as an object. More importantly, as a reviewer noted, the debate
*need not* turn on the notion of an object: it can proceed with
quantifiers, for example. The disagreement then would arise from
divergent interpretations of those quantifiers.



Putnam's pluralism has provoked very different reactions from
realists. Some argue that conceptual pluralism is consistent with
realism [Lynch 1998; Horgan and Timmons 2002; Sosa 2003], others take
Putnam's pluralism to amount to the claim that ontological
expressions are either indeterminate or that alternative ontologies
are equally good, both alternatives being problematic [Eklund 2008].
Realists cannot make sense of the Carnapian idea that existence and
truth are relative to a conceptual scheme [Brueckner 1998]. Peter van
Inwagen provides a trenchant criticism of Putnam's claims [van
Inwagen 2002]: 



> 
> I cannot grant that 'Carnap's' [DK: Ernie's]
> and 'The Polish logician's' [DK: Maxi's]
> descriptions are equally good or equivalent descriptions of the
> population of a world [DK: e.g. the contents of Ernie's
> bag]--not at least if Carnap's description is 'a
> world that contains three mereological simples and nothing
> else'. I cannot grant that they *could* be 'equally
> good or equivalent descriptions of the population of a world'
> since they are straightforwardly incompatible.
> 



Recently, however, some impressive neo-Carnapian defences of
conceptual pluralism have been proposed that bring new considerations
to bear on these debates. We briefly review some of these in section
5.


## 5. Realism and Anti-Realism in Meta-Ontology



Debates in meta-ontology (analytic ontology) over the last twenty
years have sparked renewed interest in realism. They have also seen a
marked shift in how realism, i.e. ontological realism, is understood.
"The central question of metaontology", Theodore Sider, a
prominent ontological realist, contends, "is that of whether
there are many equally good quantifier meanings, or whether there is a
single best quantifier meaning." [Sider 2009, p.397]. Where
Sider argues for a single best quantifier meaning, Eli Hirsch believes
there are a multiplicity of possible quantifier meanings that are
equally good, a thesis he calls *Quantifier Variance*. This
meaning-theoretic focus is something
 new.[10]




It is no surprise, then, to find that the positions marked out as
'realist' and 'anti-realist' by those engaged
in ontological disputes do not always coincide with realism and
anti-realism as we have explained these metaphysical
 views.[11]



A more significant division is between metaontologists who accept a
*robust* conception of ontology, and *deflationists* about
ontology who don't. Sider defends robust ontology [Sider 2009,
385-386]:



> 
> "ontological deflationists"... have said ... when some
> particles are arranged tablewise, there is no
> "substantive" question of whether there also exists a
> table composed of those particles. There are simply different --
> and equally good -- ways to talk. I, on the other hand, accept a
> very strong realism about ontology. I think that questions about the
> existence of composite objects are substantive, just as substantive as
> the question of whether there are extra-terrestrials.
> 



Neo-Carnapians such as Putnam, Eli Hirsch, David Chalmers, Amie
Thomasson, and Huw Price are ontological deflationists who embrace
conceptual pluralism about ontological matters. Hirsch, however,
thinks conceptual pluralism is perfectly consistent with realism
[Hirsch, E. 2002]. Matti Eklund understands Hirsch to mean that he
considers the world to be an amorphous lump [Eklund, M., 2008] (citing
Michael Dummett), a 'lump' that alternative and equally
feasible conceptual schemes serve to make intelligible. For Sider, in
contrast, rejecting an intrinsic structure to the world is to reject
 realism.[12]




Competing views about temporal persistence do not seem to be semantic
in nature. While Perdurantists believe that things persist through
time by virtue of having temporal parts that *perdure*,
Endurantists reject the notion of temporal parts as incoherent
--things persist by *enduring*: they are wholly present
whenever they exist. As observed in the entry on
 temporal parts:



> 
> This looks like a straightforward ontological disagreement, a dispute
> about what exists.
> 



Eli Hirsch is not convinced, however:



> 
> I claim that the dispute between Endurantists and Perdurantists is
> verbal ... each party ought to agree the other party speaks a truth in
> his own language. [Hirsch 2011, 229]
> 



How can this be? Endurantists think perdurantists are guilty of
spatializing time when they talk about temporal parts; perdurantists
think enduring objects cannot explain change. How can there be a
rapprochement of the sort Hirsch has in mind?



Hirsch's novel and intriguing idea is that what makes the
endurantist/perdurantist temporal parts debate and the
nihilist/universalist mereological debate merely verbal ones is the
fact that the protagonists in these debates mean different things by
their quantifiers, in particular their existential quantifiers, in
their ontological assertions. While both protagonists speak a common
language, here English, in which certain ontological claims such as
'there are tables' happen to come out true, this is a
superficial socio-linguistic fact about English that might not have
been so: we and they could just as easily have spoken English\* (an
ontological nihilist language) in which the sentence 'there are
tables' came out false. Protagonists in these ontological
disputes are, unwittingly, engaged in a 'merely verbal'
debate and are thus talking past each
 other.[13]




How does the deflationist tell that an ontological dispute is a
'merely verbal' one? Hirsch thinks that when we interpret
the words of another, we assign truth-conditions to their sentences by
matching those sentences with sets of possible worlds, guided by the
metasemantic maxim that the speaker's assertion of those
sentences should come out true. The maxim applies to ontological
disputes such as the Ernie/Maxi dispute about mereology -- there
are possible languages in which both speakers' assertions come
out true. Hirsch contends: "speakers of either language should
allow that speakers of the other language assert sentences that have
the same characters (DK: functions from contexts of utterance to
truth-conditions) and the same truth-values as they themselves
assert." [Hirsch 2009, p.242]. 



In this way Ernie should attribute the same set of possible worlds
(intensions) to Maxi's sentence 'There are seven objects
in your bag' as he associates with his own sentence 'There
are three objects in my bag' and Ernie should interpret Maxi as
uttering a truth in so doing. Ernie and Maxi are asserting the very
same proposition but are using different words to express it. They
are, as a result, simply talking past each other. 



Hirsch's doctrine of quantifier variance QV dominates current
metaontological debate. Some have questioned whether interpreters on
one side of an ontological dispute can admit that the language of
those on the other side is possible. For to do so each interpreter
must be able to provide a Tarskian semantics for the other's
language. But an Endurantist won't be able to do this for the
Perdurantist's sentence such as 'Alicet is a
temporal part of Alice' since the predicate 'is a temporal
part of' has an empty extension in the Endurantist's
language [Hawthorne 2006; Eklund 2009]. Others suspect QV is an
internally unstable position: how can an Endurantist speaking her
language E allow that a sentence like 'Alicet is a
temporal part of Alice' is a *true* sentence of the
Perdurantist's language P without admitting that there are
temporal parts [e.g. Hale and Wright 2009; Dorr 2014]? Warren 2015
provides a convincing QV response to this 'Collapse'
argument. An important resource, containing papers by some of the
authors cited, is the collection of essays anthologised in Chalmers et
al 2009 [For background on mereology see the entry
 mereology
 and for discussion of whether there are composite objects, see the
entry
 material constitution
 and the entry
 ordinary objects].
 



The meaning-theoretic focus on Quantifier Variance in metaontology
represents a fascinating development. The implications for ontological
realism are as yet undecided.


## 6. Summary



We have considered a number of challenges to realism, the thesis that
the objects and properties that the world contains, its nature and
structure, exist independently of our conception or perception of
them. Historically, these challenges came from two camps: (1)
neo-verificationists led by Dummett who assimilate belief in
mind-independent world to a belief in a verification-transcendent
conception of truth which they profess to find unintelligible, and (2)
pragmatists and pluralists led by Putnam who also question the
intelligibility of the realist's mind-independent world but for
reasons independent of any commitment to verificationism. While
neo-verificationism today claims few adherents, within the ranks of
analytic ontologists, pluralism and Carnap's version of it in
particular, has enjoyed something of a revival. Today, the most active
and engaging debates about realism are meta-ontological ones that
involve neo-Carnapian pluralists and their ontological realist
opponents. 



Both the historical debate between realists and their anti-realist
opponents and the meta-ontological debate are still very much open. If
realists could provide a plausible theory about how correspondences
between mental symbols and the items in the world to which they refer
might be set up, many of these challenges could be met. Alternatively,
if they could explain how, consistently with our knowledge of a
mind-independent world, no such correspondences are required to begin
with, many of the anti-realist objections would fall away as
irrelevant. In the absence of such explanations it is still entirely
reasonable for realists to believe that the correspondences are in
place, however, and there can, indeed, be very good evidence for
believing this. Ignorance of Nature's reference-fixing mechanism
is no reason for denying it exists.