For this exposition to follow the logical order of ideas, the discussion on materialism should be immediately succeeded by that on parallelism. These two doctrines are near akin; they resemble each other as the second edition of a book, revised and corrected, resembles the first. Parallelism is the materialist doctrine of those forewarned folk, who have perceived the errors committed and endeavour to avoid them, while cherishing all that can be saved of the condemned doctrine. That which philosophers criticised in materialism was the misunderstanding of the principle of heterogeneity. The parallelists have seen this mistake, and have taken steps to respect this principle: we shall see in what way. They are especially prudent, and they excel in avoiding being compromised. They put forth their hypothesis as a provisional one, and they vaunt its convenience. It is, say they, a practical method of avoiding many difficulties; it becomes for philosophers an equivalent of that phrase which so many timorous ministers repeat: "Above all, no scrapes!" Let us study the exact point on which parallelism has amended materialism. We have seen that every materialist doctrine is the expression of this idea, that physical phenomena are the only ones that are determined, measurable, explicable, and scientific. This idea does wonders in the natural sciences, but is at fault when, from the physical, we pass into the moral world, and we have seen how the materialistic doctrine fails when it endeavours to attach the physical to the mental. There are then two great difficulties which the materialistic explanation finds before it; one is a difficulty of mechanism and the other of genesis. By connecting the mind with the brain, like a function to its organ, this doctrine seeks to solve these two problems, and with what little success we have seen. Parallelism, has tried to avoid these two problems; not only does it not solve them, but it arranges so as not to propound them. The expedient adopted consists in avoiding the meeting of the physical and the mental; instead of placing them end to end and welding one to the other, they are placed in parallel fashion side by side. To explain their correlation, which so many observations vaguely demonstrate, the following hypothesis is advanced. Physical and psychical life form two parallel currents, which never mingle their waters; to every state of definite consciousness there corresponds the counterpart of an equally definite state of the nerve centres; the fact of consciousness has its antecedents and its consequences in the consciousness; and the physical fact equally takes its place in a chain of physical facts. The two series are thus evolved, and correspond strictly to each other according to a necessary law; so that the scholar who was perfectly instructed, and to whom one of these states was presented, could describe its fellow. But never does any of the terms of one series influence the terms of the other.