The perennial philosophy is the claim that the world's religious traditions, beneath their differing doctrines, rituals and institutions, express a single underlying truth, most clearly accessible in their mystical strands. The phrase philosophia perennis was used by Agostino Steuco in 1540 and taken up by Leibniz, and it was given wide currency by Aldous Huxley's anthology The Perennial Philosophy (1945).
Its core claims are that there is a divine Reality underlying the world of things; that something in the human person is akin to, or identical with, that Reality; and that the purpose of human life is to realise it, and that these claims are found, in different vocabularies, across the traditions. The Traditionalist School of Rene Guenon, Ananda Coomaraswamy and Frithjof Schuon developed a more systematic version, distinguishing an exoteric level at which religions genuinely differ from an esoteric level at which they converge, and holding the outer differences to be providential rather than accidental.
The position has had wide appeal and considerable influence, on the academic study of mysticism, on interfaith work, and on a very large amount of popular spirituality, and it has been substantially rejected by scholars of religion on grounds worth stating precisely.
The central objection is that mystical experience is not independent of doctrine. Steven Katz's constructivist argument, in a series of influential essays from 1978, holds that there is no unmediated experience: a Buddhist has Buddhist experiences and a Christian Christian ones, because the concepts, expectations and training a practitioner brings are constitutive of what is experienced rather than a description applied afterwards. On this account the reported similarity is manufactured, by reading the reports selectively and by translating unlike things into a shared vocabulary.
A second objection is that the traditions disagree about what is realised, a personal God, an impersonal absolute, an omniscient liberated soul, or the cessation of a self that was never there, and that these are not alternative descriptions of one thing but incompatible claims which the traditions themselves have argued over at length and with precision. To harmonise them is to override each tradition's own account of what it is saying.
A third objection is political: that perennialism generally takes one tradition's categories as the universal ones, and that its neutrality is apparent rather than real.
Defenders reply that some family resemblance across contemplative traditions is genuinely observable; that constructivism cannot be pressed so far as to deny any shared human capacity without becoming implausible; and that the perennialist claim concerns depth rather than doctrinal agreement. The debate is unresolved, and it matters more than most because a great deal of popular writing about religion assumes the perennialist position without ever arguing for it.