Having said this, let us compare the psychological law and that of nature. Are they identical? We shall be told that they are not, since, as a fact, errors are committed at every moment by the sudden failures of human reason. This is the first idea which arises. Human error, it would seem, is the best proof that the two laws in question are not alike, and we will readily add that a falling stone does not mistake its way, that the crystal, in the course of formation does not miss taking the crystalline shape, because they form part of physical nature, and are subject in consequence to its determinism. But this is faulty reasoning, and a moment of reflection demonstrates it in the clearest possible manner; for adaptation may miss its aim without the being who adapts himself and his surroundings necessarily obeying different laws. When the heat of a too early spring causes buds to burst forth prematurely which are afterwards destroyed by frost, there is produced a fault of adjustment which resembles an error of adaptation, and the bringing forward of this error does not necessarily imply that the tree and the whole of physical nature are obeying different laws. Moreover, the difference between the laws of nature and those of the understanding does not need deduction by reasoning from an abstract principle; it is better to say that it is directly observable, and this is how I find that it presents itself to us. The essential law of nature is relatively easy to formulate, as it is comprised in the very definition of law. It simply consists in the sentence: uniformity under similar conditions. We might also say: a constant relation between two or several phenomena, which can also be expressed in a more abstract way by declaring that the law of nature rests on the combination of two notions, identity and constancy. On the other hand, the laws of our psychical activity partly correspond to the same tendencies, and it would be easy to demonstrate that the microcosm of our thoughts is governed by laws which are also an expression of these two combined notions of constancy and identity. It is, above all, in the working of the intellectual machine, the best known and the most clearly analysed up till now, that we see the application of this mental law which resembles, as we say, on certain sides, the physical law: and the best we can do for our demonstration will doubtless be to dissect our reasoning powers. Reason, a process essential to thought in action, is developed in accordance with a law which resembles in the most curious manner a physical law. It resembles it enough to imitate it, to conform to it, and, so to speak, to mould itself on it.