[The] perennial philosophy . . . is the gold within the sectarian dross of every great religion. —Alan Watts 
The “perennial philosophy” or “perennial tradition” is a term that has come in and out of popularity in Western and religious history, but has never been dismissed by the Catholic Church. In many ways, it was actually affirmed at the Second Vatican Council in their forward-looking documents on ecumenism (Unitas Redintegratio) and non-Christian religions (Nostra Aetate). The Church, like the perennial tradition, recognizes that there are some constant themes, truths, and recurrences in all of the world religions.
In Nostra Aetate, for example, the Council Fathers begin by saying that “All peoples comprise a single community and have a single origin [created by one and the same Creator God]. . . . And one also is their final goal: God. . . . The Catholic Church rejects nothing which is true and holy in these religions.”  The document goes on to praise Native religions, Hinduism, Judaism, Buddhism, and Islam as “reflecting a ray of that truth which enlightens all people.”  This must have taken great courage and brilliance to write this in 1965 when very few people thought that way.
For as long as there have been humans on Earth, it seems we have struggled with the problem of unity and diversity. The dualistic mind, which most of us were taught to emphasize, is incapable of creating unity. It “smartly” divides reality into binaries. It cannot help but choose sides. Can you think of an era, nation, religion, or culture in which the majority has not opposed otherness? When there was no obvious “other” around (for example, sinners, Jews, or Muslims), Christianity divided itself into warring groups calling each other heretics. Yet underneath the very real differences between religions and peoples lies a unifying foundation. I see that unifying foundation as the continual bubbling up of certain constants in all of the world religions, or if you will, the perennial tradition.
Just what is this perennial tradition? I like British philosopher Aldous Huxley’s (1894-1963) description as the combination of a spiritual metaphysics, a recurring psychology of the human person, and an ethic or morality that flows from these two:
- A metaphysic which recognizes a divine Reality substantial to the world of things and lives and minds;
- A psychology that finds in the soul something similar to, or even identical with, divine Reality;
- An ethic that places man’s [sic] final end in the knowledge of the immanent and transcendent Ground of all being.
[This] is immemorial and universal. Rudiments of the Perennial Philosophy may be found among the traditionary lore of primitive peoples in every region of the world, and in its fully developed forms it has a place in every one of the higher religions. 
You might not agree with every word of Huxley’s definition or even fully understand it, but it is hard to deny that he is pointing toward truth. In fact, it sounds very similar to the first questions and answers in the old Baltimore Catholic Catechism that I studied in the 1950s.