'For that interpretation of things which is alone possible for us this is all we require to know--that the force or energy manifested, now in one way now in another, persists or remains unchanged in amount. But when we ask what this energy is, there is no answer save that it is the noumenal Cause implied by the phenomenal effect.'[39] Was it partly with Spencer in view that Mr. Bertrand Russell recently urged[40] that the word cause 'is so inextricably bound up with misleading associations as to make its complete extrusion from the philosophical vocabulary desirable'? Professor Mach[41] had previously expressed the hope 'that the science of the future will discard the use of cause and effect as formally obscure'. And as long ago as 1870 W. K. Clifford[42] tried to show in 'what sort of way an exact knowledge of the facts would supersede an enquiry after the causes of them'; and urged that the hypothesis of continuity 'involves such an interdependence of the facts of the universe as forbids us to speak of one fact or set of facts as the cause of another fact or set of facts'. Such views may, perhaps, be regarded as extreme; and the word cause is not likely to be extruded from the vocabulary of current speech, of the less exact branches of science, or of general discussions of world-processes. Still, a philosophy of science must take note of this criticism of the use of a term which is, to say the least of it, ambiguous. We must at any rate try to get rid of ambiguity. Now we live in a world of what, in a very broad and inclusive sense, may be called things; and these things are in varied ways related to each other. (I must beg leave to assume, without discussion, that the relatedness of things is no less constitutive of the world with which a philosophy of science has to deal than the things which are in relation.) And when things stand in certain kinds of relatedness to each other changes take place. The trouble is that the kinds of relatedness are so many and the kinds of change are also so many! Spencer tried to reduce all kinds of relatedness to one quasi-mechanical type; and he signally failed--or shall I say that he succeeded only by ignoring all the specific differences on the one hand, and, on the other hand, by so smudgy an extension of the meaning of mechanical and physical terms as to make them do duty in every conceivable connexion?