The notion of “the perennial philosophy” is that all the different religions and spiritual traditions of the world are just different expressions of the same singular essence: there is one universal truth that transcends all time and space. One tradition names it God, another YHWH, another Brahman, or Great Spirit, and so on. One common metaphor is of a single mountain with different paths to the peak, or another is a single jewel with many facets, or a third is a wheel with different spokes that all lead to the same hub. A favorite quote to express this is taken from the Rig Veda: “Truth is one, sages call it by various names.” While the notion has a lengthy history – see Perennialism – in the 19th and 20th centuries it took on increasing importance, firstly because of globalization and the increasing interconnection of the world’s cultures and religions, it proposes a (nonviolent) way to make sense of the plurality of different religious claims (a difference that can be resolved violently, as with fundamentalism, by denying the truths of any other religions).
Secondly within the West science had challenged much of Western Christianity’s claims, and by extension the unscientific claims of all religions and traditions, and the perennial philosophy became a way of rationalizing all of these different claims as apparent differences due to the accidents of culture, language, and history which covered or clothed the underlying essence. In this respect mysticism is a key corollary to the perennial philosophy: the mystical experience of oneness with God (in the monotheistic traditions) or with Brahman (in Advaita Vedanta Hinduism) or of enlightenment/nirvana (in Buddhism), show a number of experiential similarities – a profound experience of the oneness of all things; that this experience cannot be expressed in words; that it has a compelling power intrinsic to it beyond rationality or theory; the experience itself is relatively brief and transitory but its significance can be life-changing and long-lasting.
William James holds to a version of this (above all evident in his book The varieties of religious experience which can be interpreted as evidence for this position); the philosopher W. T. Stace defends and upholds this view of mysticism and philosophy; Aldous Huxley published his book “The perennial philosophy” in 1945, and later his explorations of psychedelics helped support the notion that psychedelic drugs were a way of inducing mystical experience; scholar of religions Huston Smith interprets the different world religions in terms of the perennial philosophy; psychologist Abraham Maslow gives an argument for this underlying essence beneath our descriptions as naturalistic and accessible to empirical investigation in his 1964 book Religions, values, and peak experiences.
In more recent decades, both the perennial philosophy as well as mysticism have come under some critical fire. In particular as argued by scholar Steven Katz, the claims to know of some “underlying essence” beneath culture, language, and history, runs into the paradox that we can only ever know some “thing” or have an “experience” in the context of, or through the mediation of, culture, language, and history. In short there is no such thing as “mysticism”, although there are Christian mystics, Hindu mystics, Muslim mystics, and so on. To argue that their version of