One of my scholarly interests is an esoteric philosophy called perennialism. To put it briefly, perennialism is a view that there are some profound truths that have been given to mankind by the Absolute (that can be identified, for example, as the God or Śūnyatā) either by direct divine revelation, meditative realization or natural reason in ancient times. These truths have then existed among humans inside of religious, mythological and philosophical traditions.
In Western esotericism, these profound truths are called philosophia perennis and prisca theologia. The former term was first used by a Catholic and Neo-Platonic theologian Agostino Steuco in the 16th century. In the early modern period, Gottfried Wilhelm Leibniz wrote about the topic, and in the 20th century Aldous Huxley wrote a book about it, The Perennial Philosophy. In Hinduism, sanātana dharma is similar term to philosophia perennis.
In the 20th century, the term perennialism was associated with a movement established by René Guénon and Frithjof Schuon and continued by Ananda Coomaraswamy and Titus Burckhardt. Guénon noticed that the esoteric movements of the modern West do not actually hold any primordial revelation or initation, so he abandoned them and rather went to look for the perennial wisdom from traditional religions. Later, Schuon cultivated his ideas further.
Frithjof Schuon used the term sophia perennis which includes philosophia perennis and religio perennis. He can be considered as the most notable perennialist thinker and author, and he originated the purely spiritual lineage of the Guénonian movement. Its most significant living thinker is an Iranian Sufi scholar, Seyyed Hossein Nasr. There is also more politically oriented branch that can be seen as a deviation from the Guénonian spiritual philosophy.
I am also under the impression that neo-Thomist philosophers, like Frederick Copleston, and New Agers also consider perennial philosophy as part of their thinking, but I have not yet looked into that. I am probably going to do that at least what comes to the neo-Thomist interpretation. The most interesting book I have so far read about perennialism is Paths to the Heart: Sufism and the Christian East (ed. James S. Cutsinger), written in Schuonian spirit.
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