Now, this is precisely the place which the principle fills in the actual procedure of the sciences. We have absolutely no means of showing that the concrete course of Nature is strictly uniform, as has already been seen. But also, we have no need, for our scientific ends, that it should be uniform. All that we require is that natural processes, when dealt with in the bulk, should exhibit no divergence from uniform routine except such as we may neglect for the purposes of practical calculation and control of the course of events. The actual success of the empirical sciences shows that this demand for approximate uniformity is actually fulfilled with sufficient closeness for all our practical purposes. That it would be so fulfilled we could have had no theoretical means of divining before putting it to the actual test. In this sense the principle, like that of Causality may be said to be a postulate made _a priori_ and in advance of experience. But, once more like the principle of Causality, it could not be presumed to be trustworthy unless the subsequent results of its employment vindicated it; it cannot, therefore, be _a priori_ in the Kantian sense of being known to be true independent of empirical verification.[136] This result is confirmed by consideration of the way in which the principle of uniform law is actually applied to concrete cases. Scientific laws, as we all know, are purely general and abstract. They state not what _will_ happen, but what _would_ happen providing that certain specified conditions and no others were operative in determining the result. In this abstract form they are, of course, statements of exact and absolute uniformities. But in this abstract form they cannot be directly applied to the calculation of the actual course of any process. To take, for instance, an example which has been used by Professor Ward.[137] We learn in Mechanics that equilibrium is maintained on the lever when the moments of the weights about the fulcrum are equal and opposite. As an abstract generalisation this is a statement of a rigid uniformity. But in order that it may be universally true, we must suppose the conditions implied in the formulation of the proposition to be fulfilled. The lever itself must be absolutely rigid, and must be weightless; it must be of absolutely uniform structure, the fulcrum must be a mathematical point, in order that friction may be excluded, and so forth. Similarly, the weights must be thought of as mere masses without any further difference of quality, and thus only capable of affecting the lever through the one property of their weight; their attachments, again, must be of ideal tenuity, or fresh complications will be introduced. But when all these conditions have been taken into account, the principle has become so abstract as to amount to the tautology that what only operates by its mass and its distance from the fulcrum will not operate by any other property.