In any actual case, the course of events will be liable to be affected by all the conditions which had to be excluded from the abstract formulation of the principle. No actual lever will be weightless or incapable of being bent or broken; its construction will never be uniform. Actual loads, again, may influence the behaviour of a lever differently according to their bulk, their chemical composition, the nature of their attachments. At an actual fulcrum there will be some degree of friction between the lever-bar and its support, and so on. In actual fact, any or all of these circumstances may affect the behaviour of the lever bar when the loads are suspended from it. Consequently, it is quite impossible to apply the mechanical generalisation with certainty to determine the course of events in a concrete case. What holds good in this instance holds good in all similar cases of the “laws” of nature. In so far as these laws are really exact they are all hypothetical, and deal only with the problem. What would be the course of a physical sequence, assuming its complete ground to be contained in the conditions enumerated in the enunciation of the law? That is, they all, in so far as they are absolute, are different forms of the tautological proposition, that where there is nothing to make any difference between two cases, there will be no difference. But the moment we apply our laws to the calculation of the actual course of an individual process, we have to recognise that the condition for their rigid exactness is absent; in the individual process there are always aspects not comprised in the conditions for which the law was enunciated, and nothing but actual experience can inform us whether the presence of these aspects will perceptibly affect the result in which we are interested. As applied to the study of an individual process, the principle of Uniformity is thus a postulate, like the principle of Causality, which can only be justified by its actual success.