The postulate of the _constancy or uniformity of nature_ is the base of _empirical laws or concepts._ This, too, is something mysterious, before which many are ready to bow, seized with reverence and sacred terror. But that postulate is not even an hypothesis, somehow conceivable, though not yet explained and demonstrated. Ordinary thought, like philosophical thought, knows that reality is neither constant nor uniform, and indeed that it is perpetually being transformed, evolving and becoming. That constancy and uniformity, which is postulated and falsely believed to be objective reality, is the same _practical necessity_ which leads to the neglect of differences and to the looking upon the different as uniform, the changeable as constant. The postulate of the uniformity of nature is the demand for a treatment of reality made uniform for reasons of convenience. _Natura non facit saltus_ means: _mens non facit saltus in naturae cogitatione,_ or, better still, _memoriae usus saltus naturae cohibet._ [Sidenote: _Pretended inevitability of natural laws._] Another consequence of this is the inversion of the assertion (to be found everywhere in the rhetoric of the natural sciences) as to the _inexorability and inevitability_ of the laws of nature. Those laws, precisely because they are arbitrary constructions of our own and give the movable as fixed, are not only not inevitable and do sometimes afford exceptions; but there _is_ absolutely _no real fact,_ which is not an _exception_ to its naturalistic law. By coupling a wolf and a she-wolf we obtain a wolf cub, which will in time become a new wolf, with the appearance, the strength, and the habits of its parents. But this wolf will not be identical with its parents. Otherwise how could wolves ever evolve with the evolution of the whole of reality, of which they are an indivisible part? By chemical analysis of a litre of water we obtain H2O; but if we again combine H2O, the water that we obtain is only in a way of speaking the same as before. For that combining and recombining must have produced some modification (even though not perceived by us), and in any case changes have occurred in reality in the subsequent moment, from which the water is not separable, and therefore in the water itself taken in its concreteness. We could consequently give the following definition: the _inexorable_ laws of nature are those that _are violated at every moment,_ while philosophic laws are by definition those that are _at every moment observed._ But in what way they are observed cannot be known, save by means of history, and therefore true knowledge knows nothing of previsions; it knows only facts that have really happened; of the future there can be no knowledge. The natural sciences, which do not furnish real knowledge, have, if possible, even less right (if one may speak thus) to talk of previsions.