The reason is easy to discover. The object of religion is transcendent; it is not a phenomenon. Now, in order to express that object, our imagination has nothing at its disposal but phenomenal images, and our understanding, logical categories, which do not go beyond space and time. Religious knowledge is therefore obliged to express the invisible by the visible, the eternal by the temporary, spiritual realities by sensible images. It can only speak in parables. The theory of religious knowledge requires for its completion a theory of symbols and symbolism. What is a symbol? To express the invisible and spiritual by the sensible and material--such is its principal characteristic and its essential function. It is a living organism, in which we must distinguish between appearance and substance. It is a soul in a body. The body is the manifestation of the soul, although it is not like it; it makes the soul active and present. The most perfect example of symbolism, in this respect, is found in language and writing--two incarnations of thought. Neither the characters formed by my pen, nor the sound made by the air in my larynx, have a positive resemblance to my thought. But these letters and sounds become signs to those who have the key to them. They express the intangible thought; they make it present and living in the minds of those who read or hear. This is still truer of the creations of art. They also are mere symbols. Art might be defined as the effort to enshrine the ideal in the real, and by a material form to express the inexpressible. This is clearly taught by the word _poesy_, which means creation. The works of great artists really live; for they have a soul, a rich and intense life, which the material form at once conceals and reveals. From architecture to music there is not an art that is not symbolical. Ethics, religion, all the disciplines relating to the subjective life of spirit, have only this means of expression. It is their peculiarity to become exterior and objective, and to dominate the external things that science studies. Symbols, much better than science, attest the victory and the royalty of spirit. If science reveals Nature, symbols make of Nature, of its transformations and its laws, the glorified image of the inner life of spirit.