"The man who wishes to know the "that" which is "thou" may se..." — Aldous Huxley quote meaning

The man who wishes to know the "that" which is "thou" may set to work in any one of three ways. He may begin by looking inwards into his own particular thou and, by a process of "dying to self" --- self in reasoning, self in willing, self in feeling --- come at last to knowledge of the self, the kingdom of the self, the kingdom of God that is within. Or else he may begin with the thous existing outside himself, and may try to realize their essential unity with God and, through God, with one another and with his own being. Or, finally (and this is doubtless the best way), he may seek to approach the ultimate That both from within and from without, so that he comes to realize God experimentally as at once the principle of his own thou and of all other thous, animate and inanimate.