## 1. Prolegomenal Considerations



Theologians often follow Immanuel Kant's example and address
various "prolegomena" or preliminary questions before
trying to do any substantive metaphysics. These include questions
about the nature of religious language and about whether or not we are
in principle able to access and understand religious truths.


### 1.1 Religious Language and Concepts



Again, we will use the term "natural theologian" to refer
to someone who aims to use ordinary human cognitive faculties (reason,
sense-perception, introspection) to establish positive truths about
the existence and nature of God and other religiously significant,
supersensible beings or states of affairs. Such a person presupposes
that sentences in human language (or at least sentences in the
language of human thought) can express some theological truths, even
if other such truths are beyond us.



Critics of natural theology sometimes challenge these semantic
presuppositions. They provide reasons to think that our thoughts,
concepts, or sentences are incapable of expressing theological truths,
because they are incapable of referring adequately to the transcendent
entities that play an important role in many religious
doctrines--entities such as Judaism's YHWH,
Neo-Platonism's One, Vedanta's Brahman, Mormonism's
Heavenly Father, and so on. The debate surrounding these issues is
often designated "the problem of religious language", but
it is usually as much about human *concepts* as it is about
sentences in natural languages. 



In the western tradition, there have been a few periods of especially
active discussion of these prolegomenal issues. The Neo-Platonic era
was one (see the discussion of negative theology in the entry on
 Plotinus),
 the high medieval period (1100-1400 or so) was another, the
Enlightenment movement in 17th-18th
century Europe (especially the empiricist portion of it) was a third.
More recent discussions have involved both analytic and continental
figures: A.J. Ayer, Ludwig Wittgenstein, Martin Heidegger, Antony
Flew, Norman Malcolm, Emmanuel Levinas, and William P. Alston have
each discussed, in very different ways, the question of whether and
how our language might succeed in referring to transcendent entities.
However, since the 1970s, analytic philosophers have turned away from
a focus on language to a revival of metaphysics, and the
"problem of religious language" has been much less
prominent.


### 1.2 Rational Access



In addition to questions about what religious language refers to and
how (if at all) religious concepts *apply*, the natural
theologian faces another set of preliminary questions about our
ability to *generate sound arguments* about such entities or
facts. Our sense-perceptual and rational faculties are clearly limited
and fallible. There are presumably many facts about the natural
universe that we are incapable of grasping due to their complexity or
inaccessibility. So why should we think that our natural faculties can
deliver truths about even more remote or transcendent entities? 



A related debate concerns whether natural theology is the only method
by which we can have access to the domain of truths about
supersensible realities of religious interests. Some practitioners
(call them *rationalists*) argue that only propositions that can
be justified by unaided human reason are candidates for permissible
belief. Others (call them *hybridists*) allow that our natural
faculties can take us a certain distance--to knowledge of the
basic nature and even existence of God, say--but argue that we
must ultimately appeal to revelation and *faith* when it comes
to more specific doctrines regarding the divine nature, acts, and
intentions. This is the canonical Roman Catholic position on faith and
reason developed in authors such as Augustine, Anselm, Aquinas, and
revived in the natural scientific context of the Renaissance by
Catalan scholar Raymond Sebond (1385-1436). Sebond's Latin
work, *Theologia Naturalis* (1434-1436), became famous
when Michel de Montaigne translated it into French in 1569 and made it
the subject of the longest of his renowned *Essays*
('Apology for Raymond Sebond'), in 1580.



There are many kinds of hybridists: while they all think a turn to
faith is necessary at some point, some seek to establish little more
than the bare existence of God before turning to faith for greater
details. Others think it is possible to develop a more robust
understanding of God from within the bounds of reason and
sense-perception. Indeed, in recent years the so-called
"ramified natural theology" movement has sought to use our
natural faculties to demonstrate (or show to be highly probable)
robust doctrines that go well beyond bare theism--for example,
specifically Christian doctrines such as that of the Trinity, the
resurrection, or the historical authenticity of certain miracles or
biblical prophecies (Swinburne 2003; Newman et al. 2003; Gauch Jr.
2011; see
 section 4 below).
 



*Opponents* of natural religion or theology, by contrast, deny
that reason and our other ordinary perceptual capacities can justify
religious beliefs. Some of these opponents are *fideists* (e.g.,
on some readings, Tertullian, Blaise Pascal, Pierre Bayle, J.G.
Hamann, F.H. Jacobi, and Soren Kierkegaard) who hold these same
beliefs as articles of faith rather than as teachings of reason (see
the entry on
 fideism).
 Pascal, for instance, was a preeminent mathematician with strong
interests in natural theology, but ultimately concluded (during what
he called a "night of fire" in November 1654) that unaided
reason is more likely to lead us to the false god "of
philosophers and scholars" than to the true "God of
Abraham, God of Isaac, God of Jacob". The 20th
century Reformed theologian Karl Barth opposed natural theology for
much the same reason, and made his opposition to Emil Brunner's
version of the hybridist project clear in a book titled simply
"*Nein!*" (Barth 1934). In his Gifford Lectures
(which were endowed by Lord Gifford to be a lecture series about
natural theology), contemporary theologian Stanley Hauerwas espouses a
fideistic view in the tradition of Pascal and Barth but claims
(somewhat perversely) that his project (which incorporates biblical
texts and specifically Christian doctrines) counts as "natural
theology" all the same (Hauerwas 2001: 15ff).



Other opponents of natural theology are *agnostics* who do not
find the fideist's turn to faith appealing. They deny that our
natural faculties succeed in justifying any positive or negative
substantial (i.e., non-analytic) theistic beliefs, and consequently
suspend belief. Agnostics differ, however, as to whether unaided
reason *could* in principle but does *not in fact*
justify such beliefs (thus Bertrand Russell's famous response to
a question about what he would say if he were to die and then confront
God on judgment day: "not enough evidence, God, not enough
evidence!"), or whether our unaided faculties are not even
*in principle* adequate to the task. 



Still other opponents of natural theology are *atheists*.
Atheists agree with fideists and agnostics that our natural faculties
cannot establish the existence of God or other religious entities. But
that's because they think those faculties provide reasons to
believe that such entities do not exist at all (see the entry on
 atheism and agnosticism).
 One such reason is the negative one that we cannot produce any sound
arguments for theistic claims. But atheists also often maintain that
there are positive reasons to believe that God does not
exist--the incoherence of the concept of God, for instance, or
the incompatibility of God's existence and the existence of
horrendous suffering and evil (see the entry on the problem of
 evil).



There are many ways to approach a survey of natural theology. Here we
have chosen to focus largely on the classic historical discussions,
and in particular on the debates in the medieval and Enlightenment
(17th-18th century) periods in the west.
We will consider versions of the two basic kinds of positive argument
in favor of religious theses: *a priori* arguments and *a
posteriori* arguments. There are species of each of these.


## 2. *A priori* arguments


### 2.1 Ontological arguments



*A priori* arguments are those that do not require an appeal to
particular sense-perceptual experiences in order to justify their
conclusions. *Immanuel Kant* gave the name
"ontological" to *a priori* arguments that aim to
prove the existence of an object from a concept or an idea of that
object (see the entry on
 ontological arguments).
 But the argument over whether such a strategy can establish the
existence of God began well before Kant's time.


#### 2.1.1 Saint Anselm of Canterbury (1033-1109)



An early and now-canonical formulation of the ontological argument is
found in the second book of St. Anselm's *Proslogion*
(Anselm 1077-78). Anselm begins by characterizing God as the
"being than which none greater can be thought" and then
seeks to show that such a being does and indeed must exist. 



Anselm's argument can be reconstructed in various ways (see the
entry on
 Saint Anselm),
 but here is one:


1. By "God" we understand something than which nothing
greater can be thought. [premise]
2. When we understand the term "God", God is in the
understanding. [premise]
3. Therefore, something than which nothing greater can be thought is
in the understanding. [by (1) and (2)]
4. What is in the understanding *and* in reality is greater
than what is in the understanding alone. [premise]
5. Therefore, God exists in the understanding and in reality. [by (3)
and (4)]



In support of (2), Anselm notes that



> 
> 
> [T]he fool has said in his heart that "There is no God".
> But when this same Fool hears me say "something than which
> nothing greater can be thought", he surely understands what he
> hears; and what he understands exists in his understanding, even if he
> does not understand that it exists [in reality]. (Anselm
> 81-2)
> 
> 
> 



So according to Anselm, even the "foolish" atheist
*understands* the term "God" when he argues that
God does not exist. By this Anselm simply means that the atheist has
the idea of God, and thus has God "in his
understanding".



Premise (4) presupposes that things can exist in a number of different
ways or modes. One of those ways is as the object of an
idea--i.e., existence "in the understanding". Another
way for it to exist is "in reality". (4) articulates a
comparative value judgment about these ways of existing: it is
*greater* for something to exist in *both* ways than it
is to exist merely in the first way. 



In order to deduce (5) from (3) and (4), Anselm uses a *reductio ad
absurdum* argument:



> 
> 
> And surely that than which a greater cannot be thought cannot exist
> only in the understanding. For if it exists only in the understanding,
> it can be thought to exist in reality as well, which is greater.
> (Anselm 82)
> 
> 
> 



More explicitly:


1. Suppose a certain being, *B*1, is conceived of
as God is in the proof, and *B*1 is in the
understanding alone. [supposition for *reductio*]
2. But we can conceive of another being, *B*2, that
is exactly like *B*1, except that
*B*2 exists in reality as well as in the
understanding. [introspection]
3. Thus, *B*2 is greater than
*B*1. [by (4)]
4. It is impossible to conceive of a being that is greater than
*B*1. [by (a) and (1)]
5. Contradiction. [by (c) and (d)]
6. Therefore, (a) is false: If *B*1 is conceived of
as God is in the proof, then *B*1 must exist in
reality as well as in the understanding. [by (e)]



Philosophers and theologians have made numerous efforts to revive or
demolish Anselm's argument over the centuries. The most
influential proponents include Rene Descartes, Gottfried
Leibniz, Charles Hartshorne, Norman Malcolm, Robert M. Adams, and
Alvin Plantinga. Its main detractors include Anselm's
contemporaneous interlocutor--a monk named Gaunilo--as well
as Thomas Aquinas, Descartes' correspondents Johannes Caterus,
Marin Mersenne, and Antoine Arnauld, Immanuel Kant, and, more
recently, David Lewis (1970), Peter van Inwagen (1977), and Graham
Oppy (1996, 2009). In what follows, a number of the relevant moves in
the early modern discussion will be considered, as well as some
contemporary developments of the 17th century modal
argument.


#### 2.1.2 Rene Descartes (1596-1650)



Descartes' ontological argument, first presented in the Fifth
Meditation, aims to prove the existence of God from the idea of God
(Descartes 1641, cited below from the edition by Adam and Tannery
(1962-1976) and referred to as "AT"). Here is one
way to formulate the argument (compare Pereboom 1996, 2010; for
alternatives see the entry on
 Descartes' Ontological Argument):


1. When I have an idea of an object, the object really has whatever
characteristics I clearly and distinctly understand it to have.
(premise)
2. I have an idea of God in which I clearly and distinctly understand
God as the being that has all perfections. (premise)
3. Therefore, God has all perfections. [by (1) and (2)]
4. Everlasting existence is a perfection. (premise)
5. Therefore, God has everlasting existence. [by (3) and (4)]
6. Therefore, God exists. [by (5)] (AT 7.63-71)



One prominent way of resisting this argument is to reduce it to
absurdity by appeal to "parity of reasoning". Johannes
Caterus, for instance, objected to Descartes that by a precisely
parallel form of reasoning we could prove the real existence of the
object of an idea of an existent lion (AT 7.99). Gaunilo's reply
to Anselm centuries earlier was similar: by parity of reasoning, one
can prove the existence in reality (and not just in the understanding)
of the maximally perfect island (Anselm 102).



The objection aims to show that, like the idea God, the idea of the
existent lion and the idea of the maximally perfect island include
existence, and thus the existence of these objects can be established
via an ontological argument. But the claim that the existence of
Caterus's lion and Gaunilo's island can be established in
this way is absurd, and thus the same holds for the theistic
ontological argument. Note that this parity argument via a
*reductio ad absurdum*, if successful, would show that the
ontological argument is unsound, but without indicating which step in
the reasoning is at fault.



A second objection, anticipated by Descartes in the Fifth Meditation,
is that truly predicating a property of something without specifying
any conditions or intentional contexts involves an affirmation that
the thing exists. So from the truth of "Macron is the President
of France", one can conclude that Macron exists. As a result,
"Pegasus is a winged horse" is strictly speaking false,
though by using the intentional context "according to the
myth" we can say, truly, "According to the myth, Pegasus
is a winged horse". This suggests that premise (3) above is
subject to a decisive challenge, and Descartes can legitimately claim
only, for instance "According to the idea of God, God has all
perfections", or "If God exists, then God has all
perfections". But then all that follows in step (6) is the
unspectacular conclusion that "According to the idea of God, God
exists", or, even less impressively, "If God exists, then
God exists". 



A third problem, raised in the Second Objections by Father Marin
Mersenne, is that the argument would be sound only if a maximally
perfect being is really possible, or, equivalently, only if there is a
genuine divine essence. But this, Mersenne complains, has not been
established (AT 7.127). (Side note: Gaunilo and Mersenne are good
examples of how devout theists might still take issue with natural
theological efforts to prove God's existence.)



Descartes' reply to these objections involves the notion of a
"true and immutable nature" ("TIN") (AT
7.101ff.). Only some of our ideas of things that have TINs. Moreover,
TINs themselves exist in some way, although they need not exist in
concrete or empirical reality. Perhaps they are abstract objects, like
numbers or sets (Descartes explicitly compares them to Plato's
Forms). In any case, the kind of existence TINs have is sufficient to
undermine the second objection above: the divine
essence--God's nature--is a *true* and
immutable nature, and thus we do not need to prefix anything like the
phrase "According to the idea of God" to premise (3).
Rather, Descartes thinks we can clearly and distinctly perceive that
God's nature is a TIN, and that this TIN contains all
perfections. Thus we can conclude that "God has all
perfections". That would make the inference to (6) a valid one.




Descartes' challenge, then, is to show that God's nature
is a TIN and that the natures of an "existent lion" and
"the maximally perfect island" are not TINs. In the Fifth
Meditation Descartes maintains that TINs are different from fictitious
ideas in that TINs are in some sense independent of the thought of
their conceivers. For example, the nature of a triangle is a TIN
because it contains properties that we don't grasp when we first
form the idea of a triangle, and deducing these further properties is
a process "more like discovery than creation". God's
nature also has this feature--we obviously don't grasp all
of the properties of the maximally perfect being when we first form an
idea of it. The problem, however, is that it is not clear how this
criterion would rule out the natures of a most perfect island or an
existent lion as TINs, since in those cases we also don't grasp
all of the properties when we first form the idea. 



Later, Descartes (AT 7.83-4) characterizes a TIN as having a
unity such that it cannot be divided by the intellect. He thinks that
having this feature shows that is hasn't been simply put
together by the intellect or imagination, and is thus a genuine
nature. Accordingly, the idea of an existent lion does not correspond
to a TIN because I can coherently conceive of a lion that
doesn't exist. Likewise I can coherently conceive of a maximally
perfect island having one fewer coconut tree but one more mango tree,
and so on. But it also seems that I can conceive of some of the divine
perfections without others (i.e. of an omnipotent being that is
lacking maximal benevolence). So by this standard it appears that the
idea of God also fails to correspond to a true and immutable nature.
Note: Descartes himself seems to resist this objection by arguing that
all of the divine perfections ultimately boil down to sovereignty or
omnipotence. 



To the third problem, concerning the real possibility of God,
Descartes replies that our clear and distinct ideas of
TINs--produced in us by reason--are reliable. Since we can
(supposedly) see clearly and distinctly that there is no contradiction
in our idea of God's nature, the denial that God is really
possible is on equal footing with the denial that the angles of a
triangle are equal to two right angles (AT 7.150-1).


#### 2.1.3 Gottfried Leibniz (1646-1716)



Leibniz addresses several of the central objections to
Descartes' ontological argument (in, e.g., Leibniz 1676 [PP]:
167-8; 1677 [PP]: 177-80; 1684 [PP]: 292-3; 1692
[PP]: 386; 1678 [PE]: 237-39; 1699 [PE]: 287-88; Adams
1994: 135-56). These include:


1. the claim that the essence of a most perfect being includes its
existence--that existence is a perfection--hasn't been
substantiated;
2. the claim that all this argument can establish is the conditional
"If an object of the concept of God exists, then God
exists"; and
3. the claim that the real possibility of a most perfect being cannot
be demonstrated



In several places Leibniz addresses (A) by arguing that by
"God" we understand a necessary being, and that from this
it follows that the essence of God involves necessary existence. In
this way we supposedly avoid altogether the premise that existence is
a perfection. (One wonders, however, whether the argument for
including "necessary existence" in the idea of God will
need to rely on the premise that necessary existence is a
perfection.)



In some writings Leibniz tries to bypass (B) by presenting an argument
with a different conditional as its conclusion (see Adams 1994:
135-42):


1. If there is a divine essence, then the divine essence involves
necessary existence. [premise]
2. If God is a possible being, then there is a divine essence.
[premise]
3. If God is a possible being, then the divine essence involves
necessary existence. [by (1), (2)]
4. If God is a possible being, then God necessarily exists. [by
(3)]
5. Therefore, if God is a possible being, then God actually exists.
[by (4)]



What has yet to be dealt with, clearly, is Mersenne's problem
above--(C), that the real possibility of a most perfect being
cannot be demonstrated. Leibniz offers several types of arguments
against this. One relies on the fact that *other* things are
clearly possible, together with the claim that only a necessary being
provides a satisfactory ground or explanation for the possible
existence of contingent beings. So on the assumption that contingent
beings possibly exist, it must be at least possible for God, as a
necessary being, to exist (for more on this kind of argument from
possibility, see section 2.3 below). 



A second type of Leibnizian argument for God's real possibility
returns to the thesis that God is the most perfect being, and adds
that perfections are positive and simple, unanalyzable qualities. So,
for example, consider any proposition of the form "*A*
and *B* are incompatible", where *A* and
*B* are any two perfections. Two properties are incompatible
only if they are logically incompatible, according to Leibniz. Thus
"*A* and *B* are incompatible" will be true
only if one of these perfections turns out to be the negation of the
other (as in *omniscient* and *non-omniscient*), or if
their analyses reveal simpler properties, one of which is a negation
of another. But on the assumption that all the divine perfections are
positive, simple, and thus unanalyzable, neither of these scenarios
can obtain. Consequently, "*A* and *B* are
compatible" is always true for any two perfections, and thus a
being with all perfections is really possible (Leibniz 1678 [PE]:
238-39; Adams 1994: 142-48). 



A third Leibnizean response to Mersenne's objection is that it
is rational to *presume* the real possibility of the things we
can conceive, at least until their impossibility has been
demonstrated.


#### 2.1.4 Immanuel Kant (1724-1804)



Kant's most famous criticism of the ontological argument is
encapsulated in his claim that "existence" (or
"exists") is not a positive determination or "real
predicate" (Kant 1781/1787: A592/B619ff). Alternatively,
"existence" is not "a predicate that is added to the
concept of the subject and enlarges it" (A598/B626; see Stang
2016 and Pasternack 2018 for discussion). Kant's idea here is
that since "existence" is not a real predicate, existing
cannot be one of God's perfections. 



One way to interpret the objection is as follows:


1. Suppose *A* and *B* are two entities, and *A*
is greater than *B* at *t*1. [premise]
2. If *B* becomes as great as *A* at
*t*2, then *B* changes. [by (1)]
3. For every entity *x*, if *x* changes, then:
	1. there is a time *t*1 at which *x* has (or
	lacks) some property *P*, and
	2. there is a later time *t*2 at which *x*
	lacks (or has) *P* as a result of *x*'s acting or
	being acted upon. [premise]
4. For every entity *x*, if *x* comes into existence in
reality, conditions (3a) and (3b) are not satisfied. [premise]
5. Therefore, when an entity comes into existence in reality, it
doesn't change. [by (3), (4)]
6. Therefore, *B* cannot become as great as *A* solely
in virtue of *B*'s coming into existence in reality. [by
(2), (5)]
7. Therefore, *A* cannot be greater than *B* solely in
virtue of existing in reality. [by (6)]



(4) is the key premise in this formulation--does coming into
existence involve a genuine change? There is clearly a technical sense
in which saying that a concept applies to something does not enlarge
the concept or change our conception of the being it refers to. But it
might remain open that a being that has all perfections but does not
exist is not as great as a being that has all perfections and also
exists.



Kant's objection can perhaps be avoided altogether by proposing
that the perfection at issue is *necessary existence*, and not
mere existence. Adding *necessary existence* to our concept of
a being would presumably involve changing it (and thus enlarging its
concept). This is effectively a modal version of the ontological
argument (see
 section 2.1.5).



Kant's most pressing criticism, in our view, goes back to the
issue raised by Mersenne: we cannot determine whether God (conceived
as having necessary existence or not) is really possible. Kant grants
to Leibniz that the notion of a most perfect being may not involve a
logical contradiction, but he argues that this is not enough to show
that it is really possible, for there are ways of being impossible
that do not involve logical contradictions (A602/B630). The
implication for the ontological argument is that we cannot know or
rationally presume that it is really possible for the divine
perfections to be jointly exemplified even if we know that they
involve no contradiction.



> 
> 
> For how can my reason presume to know how the highest realities
> operate, what effects would arise from them, and what sort of relation
> all these realities would have to each other? (Kant LPT [AK
> 28:1025-26])
> 
> 
> 


#### 2.1.5 Contemporary Modal Versions



Versions of the ontological argument discussed by Leibniz and Kant
have been elaborated by Robert M. Adams (1971), Alvin Plantinga
(1979), Peter van Inwagen (1977, 2009) and others. These versions
employ contemporary modal semantics and metaphysics to motivate the
following two assumptions:



(Assumption 1):
 "It's possible that God exists" means
"there is some possible world in which God exists".
(Assumption 2):
"God necessarily exists" means "God exists in
every possible world" (i.e., "there is no possible world
in which God doesn't exist").


The argument then proceeds as follows:


1. It's possible that God exists. [premise]
2. Therefore, God exists in some possible world (call it
*w*\*). [by (1), (Assumption 1)]
3. If God exists, then God necessarily exists. [by definition of
"God"]
4. Therefore, in *w*\* God necessarily exists. [by (2),
(3)]
5. Therefore, in *w*\* God is such that God exists in every
possible world [by (4), (Assumption 2)]
6. The actual world is one of the possible worlds [premise]
7. Therefore, in *w*\* God is such that God exists in the
actual world [by (5), (6)]
8. Therefore, God exists in the actual world [by (7)]



Critics have resisted numerous aspects of the argument (for
comprehensive discussion, see Oppy 1996). The inference to (7), for
instance, assumes that the actual world is possible *relative*
to *w*\*. But that assumption is only legitimate on some models
of how talk of modality (i.e. of possibility and necessity) works.
Thus, the critic claims, it hasn't been shown that *w*\*
has the relation to the actual world that licenses the conclusion that
God actually exists.



The most significant disagreement, however, is again the one that
Mersenne raised against Descartes. How can the claim about
metaphysical possibility in (1) be justified? Adams (1994: ch.8),
following Leibniz, claims that we are rationally permitted, in the
absence of strong reasons to the contrary, to *presume* the
real possibility of most things, including God. Others argue that no
such presumption of metaphysical possibility is justified, especially
regarding supersensible things.


### 2.2 The argument from necessary truths



Leibniz's argument for God's existence from the existence
of the necessary truths crucially involves the premises that all
truths are true in virtue of something distinct from them (they need
what contemporary metaphysicans sometimes call a
"truth-maker"). Since necessary truths would be true even
if there were no finite minds to think them, such truths cannot be
true in virtue of facts about human psychology. Against the Platonic
suggestion that they are true in virtue of Forms existing outside of
any mind whatsoever, Leibniz argues that some of the truths are about
abstract entities, which are not the kinds of things that could have
mind-independent existence. The only contender that remains, then, is
that these truths are true in virtue of the ideas in an infinite and
necessarily existent (divine) mind (Leibniz 1714 [PP]: 647; Adams
1994: 177ff.).



Leibniz's argument stakes out one position on the grounding of
necessary truths, but there are many rival views that do not invoke
the existence of a necessary and eternal divine mind. A Platonist
might respond by claiming that it hasn't been shown that
abstract entities do not have mind-independent existence, while
Humeans would argue that necessary truths are all analytic, and that
therefore only the structure of language or of our conceptual schemes
is required to ground their truth.


### 2.3 The argument from possibility



A third sort of *a priori* proof argues from facts about the
mere possibility of something (or of some collection of things) to the
existence of a "ground of possibility" that somehow
explains them. An early version of such an argument can be found in
Augustine and the Neo-Platonic tradition, but the canonical
presentations of this sort of "possibility proof" are in
Leibniz's *Monadology* (1714) and, much more elaborately,
in Kant's book-length treatise called *The Only Possible
Basis for a Demonstration of the Existence of God* (1763). Note
that if truths about what is possible are necessary truths, as they
indeed seem to be, then this might be a more specific version of the
argument from necessary truths (see
 2.2).



Kant's version of the argument is based on the claim that there
are real possibilities that are not grounded in the principle of
non-contradiction, but that nevertheless must have a ground or
explanation (Kant 1763 [AK 2: 63ff]). If they are not grounded in the
principle of non-contradiction, then they are not grounded in
God's thinking them rather than their negations, which is how
Leibniz had proposed that all such possibilities are grounded. Put the
other way around, Kant thinks that some *impossibilities* are
grounded *not* in the law of non-contradiction, but in a
non-logical kind of "real repugnance" between two or more
of their properties. Kant maintains, for instance, that it is
impossible for a material being to be conscious, even though no
logical contradiction exists between *material* and
*conscious* (Kant 1763 [AK 2: 85-6]). It follows that
this impossibility is not grounded in divine thought, for God can
think any proposition that does not involve a contradiction. So facts
about some real possibilities and real impossibilities can only be
grounded in a necessary being that somehow *exemplifies*
(rather than merely thinking) every combination of fundamental
properties whose joint exemplification is really possible. That being,
of course, is supposed to be God. (For discussion, see Fisher and
Watkins 1998; Adams 2000; Chignell 2009, 2012, 2014; Stang 2010; Abaci
2014; Yong 2014; Hoffer 2016; Abaci 2019; Oberst 2018, 2020).



One way out of this argument is simply to claim that some real
possibilities are primitive or ungrounded. This appears to be an
option for Kant in his critical period insofar as he is no longer
committed to rationalist principles such as the Principle of
Sufficient Reason (see Abaci 2019). Still, even in his critical period
(i.e., after 1770 or so), Kant never repudiated the earlier
proof--and supposedly claimed in a lecture from the 1780s that it
"can in no way be refuted" (Kant LPT [AK 28: 1034]). This
suggests that in his critical period Kant still held that God's
existence is the only available ground for real possibility, but that
the explanatory point may justify at most a kind of theoretical belief
(*Glaube*) rather than full demonstrative knowledge (see
Chignell 2009, Stang 2016, and Oberst 2020).


## 3. *A posteriori* Arguments


### 3.1 The classical cosmological argument



An *a posteriori* argument involves at least one premise whose
justification essentially appeals to some sort of empirical fact or
experience. The main demonstrative *a posteriori* argument is
what Kant dubbed "the cosmological argument". It is
motivated by the familiar question "Why is there something
rather than nothing?" and goes from the empirical fact of the
existence of something (or of the cosmos as a whole, perhaps) to the
existence of a first cause or ground of that cosmos that is, at least
in part, not identical to that cosmos. Cosmological arguments are
found across almost every philosophical tradition, and find prominence
in the West in the writings of Aristotle, Avicenna,
al-Gh&amacr;z&amacr;li, Maimonides, Aquinas, Locke, Leibniz, Samuel
Clarke, and David Hume. Here we will start with Avicenna, but then
focus largely on the early modern period.



Avicenna (980-1037), whose Arabic name is Ibn Sina, sets out
cosmological arguments in a number of works, but a detailed version is
found in his *Remarks and Admonitions* (*Kitab al-Isharat wa
l-Tanbihat*) (Avicenna [ISR]; Mayer 2001). He begins with a claim
about contingently existing things: since it is possible for a
contingent thing either to exist or not to exist, whether it exists or
not hangs in the balance. But then, if a contingent thing does exist,
there must be something external to it, a cause, that accounts for its
existing rather than not. This establishes a principle crucial to the
argument: "the existence of every contingent is from other than
it."



Avicenna next considers the aggregate of all the existing contingent
individual things, the existence of each of which is accounted for by
its causal antecedents. He then proposes and evaluates four options
for accounting for the aggregate's existence. The first is that
the existence of the aggregate does not require a cause. However,
given the principle that the existence of any contingent thing must
have a cause, the aggregate would then have to exist necessarily. But
the aggregate's existing necessarily is ruled out by the fact
that all of the individuals in it are contingent things. On the second
option, the cause of the existence of the aggregate is "the
individuals all together." But then the aggregate would cause
itself to exist, which is ruled out by the principle that the
existence of any contingent thing requires a cause other than it. The
third option is that the cause of the aggregate is one of the
individuals in it. But all of those individuals are caused to exist by
other things in the aggregate, so no one individual is qualified to be
the cause of the existence of the entire aggregate. The only remaining
option is that the existence of the aggregate has a cause external to
the aggregate. Because all of the contingently existing things are in
the aggregate, the cause must be a necessarily existing thing (Mayer
2001). Avicenna is fully aware that the argument shouldn't end
here, since it must be shown that the necessarily existing thing is
God, and he provides a number of considerations in favor of this claim
(Adamson 2013).



Avicenna's cosmological argument from the contingency of the
world contrasts with another major type of argument for God's
existence in Islamic natural theology, one that aims to demonstrate
the existence of God from the beginning of the world in time.
Al-Ghazali (1056-1111), in *The Incoherence of the
Philosophers*, develops this type of argument in two steps. The
first aims to establish, against a prominent Aristotelian tradition,
that the world is not eternal but has a beginning in time. The second
step reasons that for any being that begins to exist at a time, there
must be something that determines that it comes to exist at that time.
And thus, because the world begins to exist at a time, there must be
something which determines that it comes to exist at that time. As
al-Ghazali puts it, "every being which begins has a cause for
its beginning; now the world is a being which begins; therefore, it
possesses a cause for its beginning." He then argues that it
must be God who by free choice determines that the world comes to
exist at the time it does, and we can thus conclude that God exists.
These argumentative themes are characteristic features of the
Ash'arite theological tradition in Islam; in recent times,
al-Ghazali's argument has been defended by William Lane Craig
(1979).



Leibniz's cosmological argument (Leibniz 1697 [PE]:
149-55; 1714 [PP]: 646, [PE]: 218-19) does not assume or
attempt to establish that the world, the collection of all actual
contingent beings, has a beginning in time, and in this respect it
more closely resembles Avicenna's argument than
al-Ghazali's. Leibniz argues as follows: suppose that in fact
the world has no beginning in time, and that each being in the world
has an explanation in some previously existing being(s). Two demands
for explanation might still arise: Why is there a world at all rather
than none? and: Why does this world exist and not some other world?
Neither explanation can be provided by appealing solely to entities
within the world (or within time). Leibniz's conclusion is that
there must be a being that is not merely hypothetically, but
*absolutely* necessary, and whose own explanation is contained
within itself. This being is God. (A similar cosmological argument is
advanced around the same time by Samuel Clarke (1705), see also the
entry on
 Samuel Clarke).



David Hume puts forward three main objections to the type of
cosmological argument offered by Leibniz and Clarke (Hume 1779, Part
IX, and the entry
 Hume on religion).
 The first is that the notion of (absolutely) necessary existence
itself is problematic. Suppose that some being is absolutely
necessary--then its nonexistence should be absolutely
inconceivable. But, says Hume, for any being whose existence we can
conceive, we can also conceive its nonexistence, and thus it
isn't a necessary being. Hume anticipates the objection that if
we truly understood the divine nature, we would be unable to conceive
God's nonexistence. He replies that an analogous point can be
made about matter: for all we know, if we truly understood the nature
of matter, we would be unable to conceive its nonexistence. This would
show that the existence of matter is not contingent after all, and
that it does not require an external explanation. Thus, by parity, the
cosmological argument does not establish that God is the necessary
being who is responsible for the rest of the cosmos.



Hume's second objection is that God cannot be the causal
explanation of the existence of a series of contingent beings that has
no temporal beginning, since any causal relation "implies a
priority in time and a beginning of existence" (Hume 1779, Part
IV). In reply, it seems quite possible to conceive of a non-temporal
causal relation, and thus to conceive of God, from outside of time,
causing a series of contingent beings that has always existed. Indeed,
this view is common in the theological tradition. Moreover, as Kant
along with numerous contemporary metaphysicians argue, we can
coherently conceive of a relation of *simultaneous causation*.
If this is right, then even a God who is in time could ground the
existence of a series of contingent beings with no temporal beginning.




Hume's third objection is that in a causal series of contingent
beings without a temporal beginning, each being will have a causal
explanation by virtue of its predecessors. Since there is no first
being, there will be a causal explanation for every contingent being
on the basis of previously existing contingent beings. However, if
each individual contingent being has a causal explanation, then the
entire causal series has an explanation. For wholes are nothing over
and above their parts:



> 
> 
> did I show you the particular causes of each individual in a
> collection of twenty particles of matter, I should think it very
> unreasonable should you afterwards ask me what was the cause of the
> whole twenty. (Part IV) 
> 
> 
> 



A reply to this last objection might be that even if one has explained
in this way the existence of each individual in the contingent series,
one still has not answered the two questions mentioned earlier: Why is
there a world at all rather than none? and: Why does this world exist
and not some other world? (for further discussion, see Rowe 1975;
Swinburne 2004; Pruss 2006; O'Connor 2008).



Kant, too, objects to the cosmological argument, but mainly on the
grounds that it delivers an object that is inadequate to the classical
conception of God. Any effort to turn the ultimate ground into the
most perfect of all beings, Kant says, will have to smuggle in some
sort of ontological argument (see Pasternack 2001; Forgie 2003; Proops
2014; and the entry on
 Kant's philosophy of religion).



In general, objections to the cosmological argument (both historical
and contemporary) take one of the following forms:


1. not every fact requires explanation, and the fact that the
cosmological arguer is pointing to is one of those;
2. each being in the cosmos has an explanation, but the cosmos as an
entire series does not require an *additional* explanation,
over and above the explanations of each member of the series;
3. the sort of explanation required to explain the empirical data
cited by the cosmological arguer does not amount to a supernatural
explanation (e.g., the Big Bang could suffice);
4. the sort of explanation required to explain the empirical data
cited by the cosmological arguer is not going to deliver anything as
august as the God of traditional religious doctrine but rather, in
Hume's terms, a somewhat "mediocre deity".


### 3.2 Teleological or design arguments



The Greek word "telos" means "end" or
"purpose". The *a posteriori* arguments in natural
theology that are referred to as "teleological" claim that
the natural world displays some sort of purposive or end-directed
design, and that this licenses the conclusion that the natural world
has a very powerful and intelligent designer (see the entry on
 teleological arguments for God's existence).
 Earlier authors dubbed this sort of non-demonstrative, inductive
argument a "physico-theological" argument (see, e.g.,
William Derham 1713). 



Teleological arguments can be found in numerous traditions and time
periods, including the classical Greek and Roman context (see Sedley
2008) and the Indian philosophical tradition (see Brown 2008). In the
west the argument is primarily associated with William Paley
(1743-1805), although in fact this type of argument was
discussed by numerous early modern figures before him (see Taliaferro
2005, DeCruz and DeSmedt 2015). The fact that Paley's 1802 book
was called *Natural Theology* is no doubt part of why natural
theology as a whole is sometimes equated with the *a
posteriori* investigations of nature for the purposes of
supporting religious theses. In the analogy that made Paley's
argument famous, the relationship between a watch and a watch-maker is
taken to be saliently similar to the relationship between the natural
world and its author. If we were to go walking upon the heath and
stumble upon a watch, a quick examination of its inner workings would
reveal, with a high probability, that "its several parts were
framed and put together for a purpose" by what must have been
"an intelligence" (1802: 1-6). Likewise with the
universe as a whole.

 Earlier teleological arguments can be found
in the works of post-Cartesian atomists like Pierre Gassendi,
Cambridge Platonists like Walter Charleton and Henry More, and
mechanists like Robert Boyle. Charleton, for instance, argues in his
*The Darkness of Atheism Dispelled by the Light of Nature: A
Physico-Theological Treatise*  (1652) that the modern rejection of
Aristotelianism establishes an even greater need to appeal to a
designer to explain how inert atoms under mechanical laws can be
fashioned into an intelligible and purposive order (see Leech 2013).


A different kind of teleological argument is developed by George
Berkeley (1685-1753), for whom natural, physical objects do not
exist independently of minds, but consist solely in ideas. Given the
regularity, complexity, and involuntariness of our sensory ideas,
their source (Berkeley argues) must be an infinitely powerful,
benevolent mind that produces these ideas in us in a lawlike fashion.
God's existence can also be demonstrated from the harmony and
beauty that the ideas of the world display (Berkeley 1710: SS146).
Since according to Berkeley our ordinary experience is a type of
direct divine communication with us, our relationship with God is in
this respect especially intimate. Thus he frequently remarks, quoting
St. Paul, that "in God we live and move and have our
being" (*Acts* 17:28).


#### 3.2.1 Hume on the teleological argument



Hume's *Dialogues Concerning Natural Religion* pre-date
Paley, of course, but they feature an especially influential and
elegant critical discussion of teleological arguments (1779), a
discussion of which Paley was no doubt aware. (In fact, Paley may have
restyled his argument as an inference-to-best explanation in an effort
to avoid some of Hume's criticisms.)



Hume's assault on the teleological argument begins by
formulating it as follows (compare Pereboom 1996, 2010):


1. Nature is a great machine, composed of lesser machines, all of
which exhibit order (especially adaptation of means to ends).
[premise]
2. Machines caused by human minds exhibit order (especially
adaptation of means to ends). [premise]
3. Nature resembles machines caused by human minds. [by (1),
(2)]
4. If effects resemble each other, their causes resemble each other
as well. [premise]
5. The cause of nature resembles human minds. [by (3), (4)]
6. Greater effects demand greater causes (causes adequate to the
effects). [premise]
7. Nature is much greater than machines caused by human minds.
[premise]
8. The cause of nature resembles but is much greater than human
minds. [by (5), (6), (7)]
9. The cause of nature is God. [by (8)]
10. Therefore, God exists. [by (9)]



Hume's objections to this argument include the claims that the
analogies on which it is dependent are not exact, and thus that there
are alternative explanations for order and apparent design in the
universe. One response to these objections is that the teleological
argument should be conceived as an argument to the *best*
explanation, on the model of many scientific arguments. In that case
the analogy need not be exact, but might still show that a theistic
explanation is best (and again, Paley himself may have recognized
this). Hume, in the voice of his character Philo, concedes



> 
> 
> that the works of nature bear a great analogy to the productions of
> art is evident; and according to all the rules of good reasoning, we
> ought to infer, if we argue at all concerning them, that their causes
> have a proportional analogy. (Part XII)
> 
> 
> 



But Philo also affirms that we cannot infer any important similarities
between humans and the author of nature beyond intelligence, and in
particular we cannot infer some of the divine attributes that are most
important for sustaining traditional theistic religion (Part V). Most
significantly, given the evil that there is in the universe, we cannot
conclude that its designer has the moral qualities traditional
religion requires God to have (Part X). Thus, again, we are left with
a rather "mediocre deity".



One of Hume's neglected objections to the teleological argument
is that it generates an absurd infinite regress (Part IV). If order
and apparent design in the material universe are explained by divine
intelligence, what explains the order and apparent design that give
rise to intelligence in the divine mind? By dint of the reasoning
employed in the teleological argument, it would have to be a
super-divine intelligence. But what explains the order and apparent
design that give rise to super-divine intelligence? An absurd infinite
regress results, and to avoid it one might well suppose the material
world "contain[s] the principle of order within itself".




To this Hume has the theist Cleanthes reply that



> 
> 
> even in common life, if I assign a cause for any event, is it any
> objection that I cannot assign the cause of that cause, and answer
> every new question which may incessantly be started?
> 
> 
> 



This seems right: in scientific theorizing it is no decisive objection
against an explanation that it contains entities that are themselves
not fully explained. Crucial to the value of scientific explanations
is that they supply an explanatory *advance*, and we can
reasonably believe that a theory does so without our having in hand
complete explanations for all of the entities it posits. 


#### 3.2.2 The teleological argument from fine-tuning



In recent decades, some natural theologians have developed an
inductive argument for the existence of God that appeals to the fact
that the fundamental features of the universe are fine-tuned for the
existence of life. According to many physicists, the fact that the
universe can support life depends delicately on various of its
fundamental characteristics, notably the values of certain constants
of nature, the specific character of certain fundamental laws, and
aspects of the universe's conditions in its very early stages.
The core claim of the argument is that without an intelligent
designer, it would be improbable that the constants, laws, and initial
conditions were fine-tuned for life.



William Lane Craig (see e.g., Craig 1990, 2003) develops a version of
this argument, which proceeds as follows. The world is conditioned
principally by the values of the fundamental constants:


* a: the fine structure constant, or electromagnetic
interaction;
* mn/me: proton to electron mass ratio;
* aG: gravitation;
* aw: the weak force; and
* as: the strong force.



When one imagines these constants being different, one discovers that
in fact the number of observable universes, that is to say, universes
capable of supporting intelligent life, is very small. Just a slight
variation in any one of these values would render life impossible. For
example, if the strong force (*as*) were increased as much as
1%, nuclear resonance levels would be so altered that almost all
carbon would be burned into oxygen; an increase of 2% would preclude
formation of protons out of quarks, preventing the existence of atoms.
Furthermore, weakening the strong force by as much as 5% would unbind
deuteron, which is essential to stellar nucleosynthesis, leading to a
universe composed only of hydrogen. It has been estimated that the
strong force must be within 0.8 and 1.2 its actual strength or all
elements of atomic weight greater than four would not have formed. Or
again, if the weak force had been appreciably stronger, then the Big
Bang's nuclear burning would have proceeded past helium to iron,
making fusion-powered stars impossible. But if it had been much
weaker, then we should have had a universe entirely of helium. Or
again, if gravitation *aG* had been a little greater, all stars
would have been red dwarfs, which are too cold to support life-bearing
planets. If it had been a little smaller, the universe would have been
composed exclusively of blue giants, which burn too briefly for life
to develop. This gives us reason to believe that there is an
intelligent designer who fine-tuned the universe as we actually find
it.



The debate surrounding the fine-tuning argument is technically complex
(a detailed summary is available in the entry on
 fine-tuning).
 Here we discuss some of the most pressing issues.



As noted, a core claim of the argument as widely understood is that
the fine-tuning for life of the constants, laws, and initial
conditions would be deeply improbable without an intelligent designer
of the universe. One question concerns the notion of probability at
work in this claim. Contemporary accounts usually appeal to an
epistemic notion of probability (e.g., Monton 2006), by contrast with
physical and logical alternatives. On this reading, the core claim is
that fine-tuning for life without an intelligent designer is
improbable in the sense that we should not expect it without such a
being, or that without such a being we should be surprised that there
is such fine-tuning.



Given this understanding of the core improbability claim, some critics
(e.g., Carlson and Olsson 1998) have argued that the fine-tuning at
issue requires no explanation. Any specific sequence of heads and
tails in a long series of coin tosses is improbable in this sense;
that is, any one sequence would be one we wouldn't and
shouldn't expect. But no specific sequence requires an
explanation other than that it was randomly generated. So why should
the finely-tuned actual array of constants, laws, and initial
conditions require an explanation other than an appeal to randomness?
Many, however, disagree, and argue that the availability of
explanatory hypotheses with intuitive pull, such as an intelligent
designer or a multiverse (discussed below), indicates that a more
substantive explanation for fine-tuning is required (e.g., Leslie
1989).



A number of critics press the objection that we should not be
surprised that we observe features of the universe that are required
for our own existence. If the constants, laws, and initial conditions
were incompatible with our existence, we would not be here to observe
this. In Elliot Sober's (2003) analysis, what's at work
here is the observer selection effect, which tends to result in a
certain bias. In this case, our observations are biased toward
fine-tuning because we would not have existed to make these
observations had the universe not been fine-tuned for life. But any
bias resulting from an observer selection effect should be factored
out and set aside, and this is so for the fine-tuning at issue.



Critics of this line of reasoning cite examples in which it appears to
goes awry. In John Leslie's (1989) example, you are dragged
before a firing squad of 100 trained marksmen. The command is given;
you hear the deafening sound of the guns. But then there you are,
surprised to be observing the aftermath, and that you are still alive.
Now consider the hypothesis that the marksmen intended to miss, which
seems a reasonable option for explaining why you're still alive.
But notice that this case features an observation selection effect
analogous to the one Sober proposes for fine-tuning: you can't
observe the aftermath unless you survive the firing squad. On
Sober's recommendation, observation selection effects should be
factored out and set aside. But on this recommendation your still
being alive wouldn't require an explanation, and we
wouldn't have reason to accept the explanation that the marksmen
intended to miss. So, according to the critics, there must be
something wrong with Sober's analysis.

 Leslie contends you
should in fact be surprised that you observe that you are still alive.
That you are alive is in the relevant sense epistemically improbable,
and requires an explanation. Similarly, we should be surprised that we
observe that the universe is fine-tuned for life, and this also
requires an explanation. One way to account for Leslie's
contention is that the epistemic probability of fine-tuning should be
judged from the point of view of a reasoner who brackets or sets aside
that she is alive (and that there is any life at all), and then asks
whether we should expect that the universe is fine-tuned for life, or
whether this should be surprising (see Howson 1991, Collins 2009, and
Kotzen 2012 for views of this sort). Similarly, in the firing-squad
case, I should judge whether my surviving is epistemically probable
from a point of view in which I bracket or set aside that I am still
alive, and then ask whether I should expect that I am still alive or
whether this should be surprising.


Leslie (1989) proposes that the hypothesis of multiple universes,
i.e., of the multiverse, would provide an explanation for fine-tuning
for life. Here is his analogy (which involves guns again, oddly). You
are alone at night in an extremely dark forest when a gun is fired
from far away and you are hit. If you assume that there is no one out
to get you, this would be surprising. But now suppose that you were
not in fact alone, but instead part of a large crowd (which you do not
see because it's so dark). In that case, Leslie suggests, you
would be less surprised at being shot, since it seems at least
somewhat likely that a gunman would be trying to shoot someone in the
crowd. Leslie suggests that this story supports the multiverse
explanation for the universe being fine-tuned for life.



Roger White (2000) contends that there is a problem with
Leslie's account. While the multiverse hypothesis would explain
why *there is some universe or other that is fine-tuned for
life*, since it would raise the probability of that hypothesis, it
would not similarly explain why *this universe is fine-tuned for
life*. He contends that supposing that the gunman was firing at
random, being part of a large crowd explains--raises the
probability--*that* *someone or other is shot*, but
not *that you are shot*. By contrast, the hypothesis that the
gunman was aiming at you, as opposed to firing at random, does explain
in this way that you are shot. Similarly, the multiverse hypothesis
explains *that there is some universe or other that is fine-tuned
for life*, but not *that this universe is fine-tuned for
life*. By contrast, in White's view the hypothesis of an
intelligent designer does explain why--raises the
probability--that this universe is fine-tuned for life. White
argues:



> 
> 
> Postulate as many other universes as you wish, they do not make it any
> more likely that ours should be life-permitting or that we should be
> here. So our good fortune to exist in a life-permitting universe gives
> us no reason to suppose that there are many universes (White 2000:
> 274; see Rota 2005 for another theistic response to the multiverse
> hypothesis).
> 
> 
> 

 See the
 fine-tuning
 entry for more detail on this exchange, and on other issues raised in
the debate.

### 3.3 Arguments from religious experience



Arguments for the existence of God from a special kind of experience
are often called "arguments from religious experience".
Some philosophers and theologians have argued that our ordinary human
cognitive faculties include what John Calvin called a special
"sense of divinity" that, when not impeded or blocked,
will deliver immediately justified beliefs about supernatural entities
(Plantinga 1981, 1984). Some such thinkers, then, might construe an
argument from religious experience as belonging to the category of
natural religion or natural theology. Others, however, insist that
"characteristic of the Continental Calvinist tradition is a
revulsion against arguments in favor of theism or Christianity"
(Wolterstorff 1984: 7; see also Sudduth 2009). In any case, most
authors who write about religious experience generally construe it as
caused by something other than our ordinary faculties and their
intersubjectively available objects, and thus do not think of it as
one of the topics of natural theology (see Davis 1989, Alston 1991,
Kwan 2006).



See the entry on
 religious experience
 for expansive discussion of this issue.


## 4. "Ramified" Natural Theology



Some supporters of natural religion or natural theology seek to use
our ordinary cognitive faculties to support theses that are more
robust and specific than those of generic or
"perfect-being" theism. This project has recently been
dubbed "ramified" natural theology by Richard Swinburne
(see Holder 2013). A few of these efforts involve *a priori*
argumentation: an ancient argument for the Trinity (found in St.
Augustine (*De Trinitate*, c. 399-419, Book IX), and
Richard of St. Victor (*De Trinitate*, c. 1162-1173,
III.1-25)) is that any supreme being would have to be a
supremely loving being, and any omnipotent supremely loving being
would ultimately have to emanate from itself something that is both
other and yet just as supreme and lovable as itself, and that a third
such person would be necessary as a kind of product of that love
between the first two. Marilyn McCord Adams's books on theodicy
(Adams 2000, 2006) and Eleonore Stump's Gifford Lectures (Stump
2010) constitute an effort to consider the *a priori* problem
of evil from within the context of a ramified (and thus in this case
more robustly Christian) kind of natural theology.



But most ramified natural theology is inductive in spirit: thus Hugo
Grotius divides his *De veritate religionis Christianae* (1627)
into a first book that deals with classical natural theology but then
later books that deal specifically with the truth of Christianity.
John Locke (1695) argued for the "Reasonableness of
Christianity" using a broadly historical, probabilistic
approach. William Paley (1794) further developed this approach, and
much more recently Swinburne argued for the conclusion that
Bayesian-style reasoning justifies belief in the resurrection of Jesus
with a probability of 97% (Swinburne 2003). Numerous other
philosophers (and also many natural scientists) appeal to historical
and scientific data as well as empirical and statistical principles of
reasoning to support the authenticity of various biblical claims, the
probability of various miracle stories, and so forth (see e.g., Olding
1990; Polkinghorne 2009; Gauch Jr. 2011). Note that this is a way of
appealing to the content of sacred texts and special revelation which
is consistent with the methods of natural theology: the prophetic or
historical claims of those texts are evaluated using public evidence
and accepted canons of inductive reasoning. Still, even optimistic
natural theologians like Locke, Paley, Swinburne, and these others
count as hybridists insofar as they think there are some important
doctrines about the divine that cannot be justified by our natural
cognitive faculties. Thus even they will at some point be willing (in
Kant's famous phrase) to "deny knowledge in order to leave
room for faith" (1787, Bxxx).