What Theosophy Is (Chapter I, page 1)

"There is a school of philosophy still in existence of which modern culture
has lost sight." In these words Mr. A.P. Sinnett began his book, The
Occult World, the first popular exposition of Theosophy, published thirty
years ago. [Namely in 1881.] During the years that have passed since then,
many thousands have learned wisdom in that school, yet to the majority its
teachings are still unknown, and they can give only the vaguest of replies
to the query, "What is Theosophy?"

Two books already exist which answer that question: Mr. Sinnett's Esoteric
Buddhism and Dr. Besant's The Ancient Wisdom. I have no thought of
entering into competition with those standard works; what I desire is to
present a statement, as clear and simple as I can make it, which may be
regarded as introductory to them.

We often speak of Theosophy as not in itself a religion, but the truth
which lies behind all religions alike. That is so; yet, from another point
of view, we may surely say that it is at once a philosophy, a religion and
a science. It is a philosophy, because it puts plainly before us an
explanation of the scheme of evolution of both the souls and the bodies
contained in our solar system. It is a religion in so far as, having shown
us the course of ordinary evolution, it also puts before us and advises a
method of shortening that course, so that by conscious effort we may
progress more directly towards the goal. It is a science, because it treats
both these subjects as matters not of theological belief but of direct
knowledge obtainable by study and investigation. It asserts that man has no
need to trust to blind faith, because he has within him latent powers
which, when aroused, enable him to see and examine for himself, and it
proceeds to prove its case by showing how those powers may be awakened. It
is itself a result of the awakening of such powers by men, for the
teachings which it puts before us are founded upon direct observations made
in the past, and rendered possible only by such development.

As a philosophy, it explains to us that the solar system is a
carefully-ordered mechanism, a manifestation of a magnificent life, of
which man is but a small part. Nevertheless, it takes up that small part
which immediately concerns us, and treats it exhaustively under three
heads--present, past and future.

A Textbook of Theosophy by C.W. Leadbeater (1912).