I will attempt to answer the questions left unanswered by Descartes. But I must premise one thing. Every definition of consciousness implies a postulate: life. Though at a pinch we can picture life without consciousness, consciousness without life is absolutely inconceivable. I do not undertake to explain what life is, any more than I attempted it above. We must take it as something given. Consciousness, then, is the subjective realization of something objective, the inward realization of something outside. If in a living being a picture of its surroundings is developed, then it absorbs something which is not a necessary part of itself. Of course, this inner image must not be understood to imply an absorption of matter. It is a process in the matter of which the living being is built up. But, all the same, the image of the outer world in the inner being does signify a penetration of the latter by the former. This image, which follows the changes of the outer world and repeats them in the inner being, is consciousness. It may be shadowy and blurred, or clear and distinct; it may in rapid succession be formed and pass away, and it can be preserved as a memory; it may reflect a greater or a lesser portion of the outer world; consciousness is accordingly duller or sharper; its contents are scant or plentiful, it retains the images of a shorter or longer series of conditions in the surrounding world. Between nutrition, which is recognized as an essential phenomenon of life, and consciousness a surprising parallelism subsists. Both consist in an absorption of the outer world by the organism; nutrition is the assimilation of matter, consciousness that of stimuli. In the process of nutrition the organism digests small quantities of the outside world; in consciousness it digests the world as a whole.