possibilities, never with the actual constitution of individual things. But all real existence is individual. To put the same thing in a different way, scientific theory deals always with those features of the _what_ of things of which we take note because of their significance for our human purposes. And in dealing with these features of things, it seeks to establish general laws of linkage between them of which we may avail ourselves, for the practical purpose of realising our various human interests. This practical motive, though often not apparent, implicitly controls our whole scientific procedure from first to last. Hence the one test of a scientific hypothesis is its success in enabling us to infer one set of facts from another set, Whether the intermediate links by which we pass from the one set to the other have any counterpart in the world of real experience or are mere creations of theory, like the “uninterpretable” symbols in a mathematical calculus, is from this point of view a matter of indifference. All we require of our hypothesis is that when you start with facts capable of experimental verification, the application of it shall lead to other facts capable of experimental verification. For this reason we may justifiably conclude that to any experience which is aware of things in their concrete individuality they must present aspects which are not represented in our scientific hypotheses, and again cannot appear to it as the precise counterpart of the schemes according to which we quite legitimately reconstruct them for the purpose of scientific investigation. We shall need to bear this in mind in future when we come to discuss the real character of what appears to us as the world of physical nature.[39]