The doctrine of association may be referred to the fundamental law that all ideas rising simultaneously or successively in the human mind, tend invariably to recall each other in the same order; this is what is called association of ideas. When any two ideas have thus been constantly associated without ever being separated (as, for instance, form and colour), they unite indissolubly and thus become necessary laws. Now, of all these necessary connections, the most universal is this: no phenomenon ever appears without having been preceded by some other phenomenon, which is always the same under the same circumstances. This law is that of causality, which is both the supreme principle and, at the same time, the result of all experience. To this doctrine of J. S. Mill and Alexander Bain our author opposes the two following objections:--1st, How does it explain the generalization? 2nd, How does it explain the necessity of the laws of the understanding? On the first point the English School appeals to a law that it calls the law of _similarity_ or faculty of identifying the like in the different. But this is indeed, strictly speaking, a fact of association? Should not association, properly understood, be reduced to the law of contiguity--that is to say, to the fact of our ideas only becoming associated through relations of time? To admit the faculty of recognizing similarity in diversity, what is this but to admit mind, intelligence--something, in short, which is other than a simple external association? As to the second point, can we reduce the rational necessity that Kant and Leibnitz have laid down as the criterion of _à priori_ principles to a pure necessity of habit--that is to say, to the automatic expectation of the future inscribed on the past? Where is the scientific guarantee in this hypothesis? Why should Nature bend to our habits? "Who can assure us that we do not dream in thinking of the future, and that the next sensation may not interrupt our dream by an unforeseen shock?" We see how far-reaching this doubt is; it affects not only metaphysic but science as well.