The perennial philosophy — the term popularized by Aldous Huxley, anticipated by Leibniz, and rooted in the Neoplatonic tradition — is the claim that beneath the enormous diversity of the world's religious traditions lies a single mystical core: the experience of an ultimate unity, a ground of being or consciousness apprehended differently by different traditions but in some sense the same reality. Its critics within each tradition rightly point out that the differences are not superficial — that Brahman is not the same as YHWH is not the same as śūnyatā — and that the claim of a hidden unity can flatten genuine distinctions. But its deepest form is not a theological claim but an experiential one: that the flame the mystics of every tradition have touched, however different their names for it, is lit from the same source.
Rumi