the subfield of philosophy devoted to the study of religious phenomena. Although religions are typically complex systems of theory and practice, including both myths and rituals, philosophers tend to concentrate on evaluating religious truth claims. In the major theistic traditions, Judaism, Christianity, and Islam, the most important of these claims concern the existence, nature, and activities of God. Such traditions commonly understand God to be something like a person who is disembodied, eternal, free, all-powerful, all-knowing, the creator and sustainer of the universe, and the proper object of human obedience and worship. One important question is whether this conception of the object of human religious activity is coherent; another is whether such a being actually exists. Philosophers of religion have sought rational answers to both questions.
The major theistic traditions draw a distinction between religious truths that can be discovered and even known by unaided human reason and those to which humans have access only through a special divine disclosure or revelation. According to Aquinas, e.g., the existence of God and some things about the divine nature can be proved by unaided human reason, but such distinctively Christian doctrines as the Trinity and Incarnation cannot be thus proved and are known to humans only because God has revealed them. Theists disagree about how such divine disclosures occur; the main candidates for vehicles of revelation include religious experience, the teachings of an inspired religious leader, the sacred scriptures of a religious community, and the traditions of a particular church. The religious doctrines Christian traditions take to be the content of revelation are often described as matters of faith. To be sure, such traditions typically affirm that faith goes beyond mere doctrinal belief to include an attitude of profound trust in God. On most accounts, however, faith involves doctrinal belief, and so there is a contrast within the religious domain itself between faith and reason. One way to spell out the contrast – though not the only way – is to imagine that the content of revelation is divided into two parts. On the one hand, there are those doctrines, if any, that can be known by human reason but are also part of revelation; the existence of God is such a doctrine if it can be proved by human reason alone. Such doctrines might be accepted by some people on the basis of rational argument, while others, who lack rational proof, accept them on the authority of revelation. On the other hand, there are those doctrines that cannot be known by human reason and for which the authority of revelation is the sole basis. They are objects of faith rather than reason and are often described as mysteries of faith. Theists disagree about how such exclusive objects of faith are related to reason. One prominent view is that, although they go beyond reason, they are in harmony with it; another is that they are contrary to reason. Those who urge that such doctrines should be accepted despite the fact that, or even precisely because, they are contrary to reason are known as fideists; the famous slogan credo quia absurdum (‘I believe because it is absurd’) captures the flavor of extreme fideism. Many scholars regard Kierkegaard as a fideist on account of his emphasis on the paradoxical nature of the Christian doctrine that Jesus of Nazareth is God incarnate. Modern philosophers of religion have, for the most part, confined their attention to topics treatable without presupposing the truth of any particular tradition’s claims about revelation and have left the exploration of mysteries of faith to the theologians of various traditions. A great deal of philosophical work clarifying the concept of God has been prompted by puzzles that suggest some incoherence in the traditional concept. One kind of puzzle concerns the coherence of individual claims about the nature of God. Consider the traditional affirmation that God is allpowerful (omnipotent). Reflection on this doctrine raises a famous question: Can God make a stone so heavy that even God cannot lift it? No matter how this is answered, it seems that there is at least one thing that even God cannot do, i.e., make such a stone or lift such a stone, and so it appears that even God cannot be all-powerful. Such puzzles stimulate attempts by philosophers to analyze the concept of omnipotence in a way that specifies more precisely the scope of the powers coherently attributable to an omnipotent being. To the extent that such attempts succeed, they foster a deeper understanding of the concept of God and, if God exists, of the divine nature. Another sort of puzzle concerns the consistency of attributing two or more properties to God. Consider the claim that God is both immutable and omniscient. An immutable being is one that cannot undergo internal change, and an omniscient being knows all truths, and believes no falsehoods. If God is omniscient, it seems that God must first know and hence believe that it is now Tuesday and not believe that it is now Wednesday and later know and hence believe that it is now Wednesday and not believe that it is now Tuesday. If so, God’s beliefs change, and since change of belief is an internal change, God is not immutable. So it appears that God is not immutable if God is omniscient. A resolution of this puzzle would further contribute to enriching the philosophical understanding of the concept of God.
It is, of course, one thing to elaborate a coherent concept of God; it is quite another to know, apart from revelation, that such a being actually exists. A proof of the existence of God would yield such knowledge, and it is the task of natural theology to evaluate arguments that purport to be such proofs. As opposed to revealed theology, natural theology restricts the assumptions fit to serve as premises in its arguments to things naturally knowable by humans, i.e., knowable without special revelation from supernatural sources. Many people have hoped that such natural religious knowledge could be universally communicated and would justify a form of religious practice that would appeal to all humankind because of its rationality. Such a religion would be a natural religion. The history of natural theology has produced a bewildering variety of arguments for the existence of God. The four main types are these: ontological arguments, cosmological arguments, teleological arguments, and moral arguments.
The earliest and most famous version of the ontological argument was set forth by Anselm of Canterbury in chapter 2 of his Proslogion. It is a bold attempt to deduce the existence of God from the concept of God: we understand God to be a perfect being, something than which nothing greater can be conceived. Because we have this concept, God at least exists in our minds as an object of the understanding. Either God exists in the mind alone, or God exists both in the mind and as an extramental reality. But if God existed in the mind alone, then we could conceive of a being greater than that than which nothing greater can be conceived, namely, one that also existed in extramental reality. Since the concept of a being greater than that than which nothing greater can be conceived is incoherent, God cannot exist in the mind alone. Hence God exists not only in the mind but also in extramental reality. The most celebrated criticism of this form of the argument was Kant’s, who claimed that existence is not a real predicate. For Kant, a real predicate contributes to determining the content of a concept and so serves as a part of its definition. But to say that something falling under a concept exists does not add to the content of a concept; there is, Kant said, no difference in conceptual content between a hundred real dollars and a hundred imaginary dollars. Hence whether or not there exists something that corresponds to a concept cannot be settled by definition. The existence of God cannot be deduced from the concept of a perfect being because existence is not contained in the concept or the definition of a perfect being. Contemporary philosophical discussion has focused on a slightly different version of the ontological argument. In chapter 3 of Proslogion Anselm suggested that something than which nothing greater can be conceived cannot be conceived not to exist and so exists necessarily. Following this lead, such philosophers as Charles Hartshorne, Norman Malcolm, and Alvin Plantinga have contended that God cannot be a contingent being who exists in some possible worlds but not in others. The existence of a perfect being is either necessary, in which case God exists in every possible world, or impossible, in which case God exists in no possible worlds. On this view, if it is so much as possible that a perfect being exists, God exists in every possible world and hence in the actual world. The crucial premise in this form of the argument is the assumption that the existence of a perfect being is possible; it is not obviously true and could be rejected without irrationality. For this reason, Plantinga concedes that the argument does not prove or establish its conclusion, but maintains that it does make it rational to accept the existence of God. The key premises of various cosmological arguments are statements of obvious facts of a general sort about the world. Thus, the argument to a first cause begins with the observation that there are now things undergoing change and things causing change. If something is a cause of such change only if it is itself caused to change by something else,