The search for truth is the noblest aim that man can pursue. The original principles of being were uncovered ages upon ages agone, and were coordinated into a complete and marvelous system. Upon that wonderful system as a basis, the teachers of ancient times laid the foundations of the superstructures of the various philosophic and religious systems which have come down to us in the world's literatures as now extant. These systems contain in greater or less degree fundamental truths of being, the study of which in our own time is called occultism -- the science of the kosmos and of man as a part thereof; telling us of the origin, the nature, and the destiny of the universe and of man, a part thereof.
The Mystery Schools of antiquity formed the inner focus of the ancient thought, and the doctrines there studied were called the heart doctrine, because they represented the doctrines that were hid; and the various philosophies that they expounded in public were called the eye doctrine, because they were the doctrines in exoteric phrasing of the things that were seen and not the things that were hid. The heart doctrine comprised the solutions of the enigmas of being, and these solutions were put forth in exoteric form under the guise of allegory and in mythological treatment, and formed the eye doctrine, or the exoteric religions or philosophies. . . . These Mystery Schools taught causal things, and their effects in life; they taught of the most intimate and profound relations of the elementary beings, whatever their degree in the kosmos, to each other. They taught man that the way to a complete understanding of the mysteries of being was by searching within; that no man could understand a truth properly which merely entered his ears, without having developed in himself also first the capacity, the inner faculty, of comprehension; and it was the development of this inner faculty of comprehension or understanding that was the main aim of the schools of initiation.
The whole effort, then, of these initiating schools was to make man know himself. Why? Because the inmost essence of man is rooted in the inmost essences of the universe, and by following the small, age-old path, as the Upanishads said, which lies within man himself, and to which Jesus alluded when he said, "I am the way, the truth, and the life" -- by following that small, old path inwards, man could climb step by step inwards or "upwards," inwards and inwards forever, constantly expanding into greater fields of consciousness. And this evolving of the inner man is achieved through the evocation of the spiritual powers latent in man's higher nature. It is, in fact, the uniting man's soul to his divine monad, his inner god.
(From Fundamentals of the Esoteric Philosophy, Theosophical University Press)