movement. For in a movement we may find the reason of another movement, but not the reason of a conscious state: only observation can prove that the latter accompanies the former. Now the unvarying conjunction of the two terms has not been verified by experience except in a very limited number of cases and with regard to facts which all confess to be almost independent of the will. But it is easy to understand why physical determinism extends this conjunction to all possible cases. [Sidenote: Physical determinism, when assumed to be universal, postulates psychological determinism.] Consciousness indeed informs us that the majority of our actions can be explained by motives. But it does not appear that determination here means necessity, since common sense believes in free will. The determinist, however, led astray by a conception of duration and causality which we shall criticise a little later, holds that the determination of conscious states by one another is absolute. This is the origin of associationist determinism, an hypothesis in support of which the testimony of consciousness is appealed to, but which cannot, in the beginning, lay claim to scientific rigour. It seems natural that this, so to speak, approximate determinism, this determinism of quality, should seek support from the same mechanism that underlies the phenomena of nature: the latter would thus convey to the former its own geometrical character, and the transaction would be to the advantage both of psychological determinism, which would emerge from it in a stricter form, and of physical mechanism, which would then spread over everything. A fortunate circumstance favours this alliance. The simplest psychic states do in fact occur as accessories to well-defined physical phenomena, and the greater number of sensations seem to be bound up with definite molecular movements. This mere beginning of an experimental proof is quite enough for the man who, for psychological reasons, is already convinced that our conscious states are the necessary outcome of the circumstances under which they happen. Henceforth he no longer hesitates to hold that the drama enacted in the theatre of consciousness is a literal and even slavish translation of some scenes performed by the molecules and atoms of organized matter. The physical determinism which is reached in this way is nothing but psychological determinism, seeking to verify itself and fix its own outlines by an appeal to the sciences of nature. [Sidenote: Is the principle of conservation of energy universal valid?]