Though Alan Watts was an Anglican minister and served as chaplain at Northwestern University, in the book he wrote in 1940 when he was just 24, The Meaning of Happiness, three Eastern religions/philosophies garner the most attention and praise.
That's because Watts correctly sees Vedanta, Buddhism, and Taoism as being transformational, rather than metaphysical, as Christianity, Judaism, and Islam are. Just picture Christianity without Jesus, or Judaism without the history of the Jewish people, or Islam without the revelation of the Koran via Mohammed.
Hard to do, if not impossible. But Vedanta, Buddhism, and Taoism do just fine without any history or personalities. Even Buddhism teaches "if you meet the Buddha on the road, kill him." Meaning, Buddha the person isn't at all necessary to understand Buddhism. Watts says:
For the wisdom of the East has a strictly practical aim which is not mere knowledge about the universe; it aims at a transformation of the individual and of his feeling for life through experience rather than belief.
This experience is psychological, not metaphysical, and except in certain specialized fields has no relation to occultism or to what we understand in the West as philosophy.
In an interesting chapter called "The One in the Many," Watts describes how the three Eastern practices aim to lead to a realization that the reality of this world partakes of the reality of all existence.
That, in a nutshell, is the open secret of happiness, according to Watts: accepting what is, here and now, for the smallest piece of the world where we live and breathe possesses the same reality as the vastest reaches of the cosmos.
The Upanishads are the foundation of what was subsequently known as Vedanta, which is to say the "end" of fulfillment of the Vedas, the Vedas being the earliest of all Hindu scriptures. With matchless economy and beauty of language the Upanishads speak in a number of different ways of Brahman, the One Reality which is expressed in all the manifold forms, objects, activities, and living beings of the universe.
...Therefore the phrase "That art thou" (tat tvam asi) is the first, fundamental principle of Vedanta arising from the second, that all things without exception are Brahman, not by participation, for Brahman is one and indivisible. Hence Vedanta is also known as the system of Advaita (literally, "not two") or nonduality, and nonduality in philosophy is the natural expression of total acceptance in psychology.
...In other words, if man has the experience of separateness, this experience also is Brahman since there is nothing other than Brahman. Maya is not illusion as against reality, for in the Vedantist conception there is nothing apart from Reality which may be set over against it. Thus we may say that while there is no actual separateness from Brahman, there is certainly an experience of separateness, and this experience is real even though it is incomplete and relative.
Buddhism sprang into being in India as a reaction to Vedanta losing its way.
But invariably there came a time when the experience was obscured by the doctrine, when the psychology of religion became the philosophy of religion and the Brahmanic tradition degenerated into scholasticism, ceremonialism, and fantastic extremes of asceticism whose object was not to accept the opposites but to destroy them, so negatively was the doctrine interpreted.
I love how Watts describes the Buddhist scriptures, which fell into much the same trap as Vedanta did.
Although we have in writing a prodigious number of words attributed to the Buddha, little is known of him.
...The earliest records of his teaching are found in the Pali Canon, three large groups of scriptures which read, for a great part, like a statistical report compiled on wet afternoons by monks who had nothing better to do.
...As for the sections on psychology, never were there such ponderous lists of minutiae, the apparent aim of which is to analyze the human being down to the last detail and so prove that he does not really exist.
On the plus side, as revealed in the Dhammapada:
The Buddha's teaching is unique in its utter lack of theology; it concentrates wholly on the necessity of arriving at a personal immediate experience and dispenses with the doctrinal symbol of that experience. In this respect it is the only truly psychological religion.
...In its most concrete form ordinary, everyday experience is just how you feel at this moment. In a certain sense Buddhism is very much a philosophy and a psychology of the moment, for if we are asked what life is, and if our answer is to be a practical demonstration and not a theory, we can do no better than point to the moment -- now!
It is in the moment that we find reality and freedom, for acceptance of life is acceptance of the present moment now and at all times.
...For this reason Mahayana Buddhism teaches that Nirvana or enlightenment cannot really be attained, because the moment we try to attain it by our own power we are using it as an escape from what is now, and we are also forgetting that Nirvana is unattainable in the sense that it already is.
Lastly, there's Taoism.
All our reasoning is based on the law of cause and effect operating as a sequence. Something is happening now because something else happened then. But the Chinese do not reason so much along this horizontal line from past, through present, to future; they reason perpendicularly, from what is in one place now to what is in another place now.
In other words, they do not ask why, or from what past causes, a certain set of things is happening now; they ask, "What is the meaning of those things happening together at this moment?" The word "Tao" is the answer to this question. The present situation within and around oneself is Tao, for the present moment is life.
...This is another way of saying that there is a harmony called Tao which blends all events in each moment of the universe into a perfect chord. The whole situation in and around you at this instant is a harmony with which you have to find your own union if you are to be in accord with Tao.
When you have discovered your own union with it, you will be in the state of Te, sometimes rendered as "virtue" or "grace" or "power," but best understood as Tao realized in man.
So it all comes down to accepting the reality of what is in the present moment. Pretty damn simple, when Vedanta, Buddhism, and Taoism are stripped down to their essence.