“Mystics understand the roots of the Tao but not its branches; scientists understand its branches but not its roots. Science does not need mysticism and mysticism does not need science; but man needs both.”

Source: The Two Sources of Morality and Religion (1932), Chapter III : Dynamic Religion
Context: Religion is to mysticism what popularization is to science. What the mystic finds waiting for him, then, is a humanity which has been prepared to listen to his message by other mystics invisible and present in the religion which is actually taught. Indeed his mysticism itself is imbued with this religion, for such was its starting point. His theology will generally conform to that of the theologians. His intelligence and his imagination will use the teachings of the theologians to express in words what he experiences, and in material images what he sees spiritually. And this he can do easily, since theology has tapped that very current whose source is the mystical. Thus his mysticism is served by religion, against the day when religion becomes enriched by his mysticism. This explains the primary mission which he feels to be entrusted to him, that of an intensifier of religious faith.

LSD: The Geek's Wonder Drug? (2006)

Our Christ : The Revolt of the Mystical Genius (1921)
Context: In point of fact there are two kinds sorts of mysticism, differing from one another as the ranting of drunkards from the language of illumined spirits. There is the muddled, stammering mysticism, and there is the mysticism luminous with truly ultimate ideas. On the one hand there are the empty dimness and darkness, the barren, chilling sentimentalism and mental debauchery, the foolishly grimacing but rigid phantasms of the Cabbala, of occultism, mysteriosophy and theosophy. We cannot draw too sharp a dividing line between these and the brightness, the simple sincerity, and healthy, rejuvenating strength of genuine mysticism, which takes the most precious gems from philosophy's treasure chest and displays them in the beauty of its own setting. Mysticism is in complete accord with the result, with the sum of philosophy. In fact, mysticism is precisely the sum and the soul of philosophy, in the form of that rapturous, passionate outpouring of love.... We are concerned with an understanding of this serious mysticism, and its meaning could be stated in three words... godlessness... freedom from the world... blessedness of soul.

1960s, Playboy Interview (1969)

My Universe (1924)

Autobiography of Values (1978)
Context: In some future incarnation from our life stream, we may even understand the reason for our existence in forms of earthly life. The growing knowledge of science does not refute man's intuition of the mystical. Whether outwardly or inwardly, whether in space or in time, the farther we penetrate the unknown, the vaster and more marvelous it becomes. Only in the twentieth century do we realize that space is not empty, that it is packed with energy; it may be existence's source. Then, if space has produced existence and the form of man, can we deduce from it a form for God?

Source: Letters to a Young Scientist (2013), chapter 5, "The Creative Process", page 69.

Source: The Bed of Procrustes: Philosophical and Practical Aphorisms (2010), p. 4