Aquinas and Bertrand Russell agreed on the rules for believing in God
Thomas Aquinas and Bertrand Russell reached opposite conclusions about God, yet they shared a principle: belief is rational only when backed by sufficient evidence. Their quarrel was over whether such evidence exists. Other philosophers reject the principle itself, and that dispute runs through the whole philosophy of religion.
The field examines religion's central ideas, such as God, souls, miracles, revelation and mystical experience, and asks whether they hang together and whether they are true. Unlike theology, it does not start from a tradition's authority, and unlike religious studies it is not limited to history or sociology; believers and non-believers practise it alike. The name only came into wide Western use in the nineteenth century, though the questions appear in the Upanishads, in Daoist, Confucian and Buddhist texts and in Aristotle, whose unmoved mover causes motion the way an object of desire does.
Religions disagree deeply about ultimate reality. Keith Yandell distinguishes three historical monotheisms: a Greek version with an eternal, uncreated world; a Semitic one in which God creates at a moment and acts in history; and an Indian one where a beginningless world is continually sustained by God. Jains see only matter and countless souls, without a supreme creator; Madhyamaka Buddhists such as Nagarjuna describe ultimate reality as emptiness; and Advaita Vedanta, defended by Adi Shankara, treats all distinct persons and gods as illusion within one quality-less Brahman.
On what counts as justified belief, evidentialism sets a strict bar. William Clifford declared it always wrong, for anyone, to believe on insufficient evidence, and William James answered in 1896 with The Will to Believe, arguing that when a live question cannot be settled by reason, belief can be justified by its practical value in a life.
Reformed epistemologists such as Alvin Plantinga go further, holding that belief in God can be properly basic, like a memory or a perception, reasonable without argument so long as it survives objections; otherwise, they say, justification faces an infinite regress. Newer work asks what it means when equally intelligent, well-informed people disagree about religion, and whether that disagreement should push anyone toward agnosticism.
Source: Philosophy of religion
