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 easier it seems to demonstrate that this union exists, the more difficult it appears to explain how it is realised; and the proof of this difficulty is the number of divergent interpretations given to it. Were it a simple question of fact, the perpetual discussions and controversies upon it would not arise.

Many problems here present themselves. The first is that of the genesis or origin of the consciousness. It has to be explained how a psychical phenomenon can appear in the midst of material ones. In general, one begins by supposing that the material phenomena are produced first; they consist, for instance, in the working of the nervous centres. All this is physical or chemical, and therefore material. Then at a given moment, after this mechanical process, a quite different phenomenon emerges. This is thought, consciousness, emotion. Then comes the question whether this production of thought in the midst of physical phenomena is capable of explanation, and how thought is connected with its physical antecedents. What is the nature of the link between them? Is it a relation of cause to effect, of genesis? or a coincidence? or the interaction of two distinct forces? Is this relation constant or necessary? Can the mind enjoy an existence independent of the brain? Can it survive the death of the brain?