On this point only two hypotheses have ever been advanced, and I think it is impossible to conceive that any third one is open. Of these two hypotheses the earliest, and of course the most obvious, is that of mental purpose. The other hypothesis is one which we owe to the far-reaching thought of Mr. Herbert Spencer. In Chapter VII of his _First Principles_ he argues that all causation arises immediately out of existence as such, or, as he states it, that 'uniformity of law inevitably follows from the persistence of force.' For 'if in any two cases there is exact likeness not only between those most conspicuous antecedents which we distinguish as the causes, but also between those accompanying antecedents which we call the conditions, we cannot affirm that the effects will differ, without affirming either that some force has come into existence or that some force has ceased to exist. If the co-operative forces in the one case are equal to those in the other, each to each, in distribution and amount; then it is impossible to conceive the product of their joint action in the one case as unlike that in the other, without conceiving one or more of the forces to have increased or diminished in quantity; and this is conceiving that force is not persistent.'