Philosophy of religion was classically regarded as part of metaphysics, since Aristotle, in some of whose writings were later identified by editors as The Metaphysics, described one of the subjects of his investigation as the investigation of first causes. For Aristotle, God was the first cause, the Unmoved Mover. Philosophy of religion as a branch of metaphysics later came to be called, by rationalist philosophers of the seventeenth and eighteenth centuries, natural theology. In the twentieth century, philosophers have adopted the name "philosophy of religion" for the subject, and typically it is regarded as a separate field of specialization, but it is also sometimes still treated as part of metaphysics, particularly by Catholic philosophers.
On nearly anyone's conception, it should be clear why considerations of the divine have been regarded as metaphysical. God, according to most conceptions of God of the divine, would be in an important category of being different from the rest of the universe. That is, God is typically conceived as not having a body, and the "mind" of the divine is not typically regarded as anything very like an ordinary human mind. Metaphysics, and in particular ontology, is concerned with the most basic categories of existence, those types of existence that cannot be explained as any other type of existence. On the view of many, the very notion of God (the gods, the divine) cannot be reduced to human concepts of mind or body; God is, on such a view, a sui generis entity, an entity in a category all of its own.
There are a lot of philosophical questions that can be asked about religious beliefs. But there are two central questions in this field. They are:
The question "What is God?" is sometimes also phrased as "What is the meaning of the word, God?" Most philosophers expect some sort of definition as an answer to this question, but they are not content simply to describe the way the word is used, they want to know the essence of what it means to be God. Western philosophers typically concern themselves with the God of monotheistic religions (see the nature of God in Western theology), but discussions also concern themselves with other conceptions of the divine.
Indeed, before attempting a definition of a term it is essential to know what sense of the term is to be defined. In this case, this is particularly important because there are a number of widely different senses of the word 'God'. The term is ambiguous: it is used in different ways by different people. So before we try to answer the question "What is God?" by giving a definition, first we have to get clear on which conception of God we are trying to define. Among those people who believe in supernatural beings, some believe there is just one God (monotheism See also monotheistic religion.), while others, in the greatest numbers Hindus, believe in many different gods. (polytheism See also polytheistic religion.) Buddhists generally do not believe in a personal God similar to that of the Abrahamic religions!
Within these two broad categories there is a huge variety of possible beliefs--although there are relatively few popular ways of believing. For example, among the monotheists there have been those who believe that the one God is like a watchmaker who wound up the universe and now does not intervene in the universe at all; this view is deism. By contrast, the view that God continues to be active in the universe is called theism.
Traditionally philosophers of religion, at least in Europe, were interested in finding out what the word "God" might refer to, in the sense in which it is used by theists. Again, theism, can be defined as the view that exactly one God exists, who is an eternally existent spirit, that exists apart from space and time, which has created the universe out of nothing, and is therefore all-powerful; and usually this being is also thought to be all-knowing and all-loving. Even once the word "God" is defined in this sense, there are still many difficult questions to be asked about what this means. For example, what does it mean for a spirit to create anything? What does "all-powerful" mean?
The second question: "Do we have any good reason to think that God exists, or to think that God does not exist?" is equally important in the philosophy of religion. Since Plato and Aristotle, philosophers and theologians have offered arguments and counterarguments for the existence of God.
Major philosophers of religion: Thomas Aquinas -- Duns Scotus -- Saint Augustine -- Saint Anselm -- Samuel Clarke -- Immanuel Kant -- Baruch Spinoza -- Soren Kierkegaard -- Maimonides -- Max Weber -- Rene Descartes -- --Abraham Joshua Heschel -- David Hume -- Anicius Manlius Severinus BoŰthius -- Charles Hartshorne -- John Hick -- J. L. Mackie -- Rudolf Otto -- Alvin Plantinga -- Richard Swinburne -- Peter van Inwagen