250 CRITICAL NOTICES: are not observed to be exclusively interested in women their junior by 2-05 years, though according to the tables this is the difference of age between the Englishman and his wife. ... I contend then that the most the physicist is entitled to assert is that, if there are molecules, the mass of the mean oxygen ' atom ' is sixteen, that of the mean hydrogen ' atom ' being taken as unity, and so on for the rest of his table of masses. He is not entitled to say that if there are molecules the mass of every oxygen atom is. precisely sixteen times the mass of any hydrogen atom." In a word, we must once more take care not to confuse calculation, which is always abstract and concerned with the hypothetical, with measurement, which is always concrete and concerned with the real world of experience. It is not however till we come to deal with the theories which, like Lord Kelvin's famous hydro- kinetic theory of matter, go the full length of regarding all sen- sible qualities as 'resulting from displacements in an absolutely homogeneous medium that we see where the consistent identifi- cation of the real with the constants of mathematical science must finally lead us. Of the hydrokinetic theories, when treated simply as a basis for mathematical calculation, it would of course be a presumptuous impertinence for the philosopher to speak, but there is no one who has a better right to pass censure on the attempts to identify the supposed "primordial fluid" with "what actually goes on behind what we can see or feel ". Against such an identification of reality with a something of which we can only say that it has none of the qualities which characterise any of the realities which we know, powerful arguments have been fre- quently raised by various philosophers, but Prof. Ward seems to have indicated the real character of the confusion more plainly than any of his predecessors. As he points out (vol. i., p. 132) the properties of the ' primordial fluid ' are simply those of space. Indeed one might go a step further ; the space from which the fluid of the. hydrokinetic theories is indistinguishable is not even the space of concrete perceptual experience. The physical world, as we experience it, contains of course a great deal more than places and movements ; it is a realm of qualitative changes which may no doubt be to a certain extent calculated beforehand with the aid of a mechanical scheme, but of which it seems unmeaning to say that they really are nothing but me- chanical movements. But even the space that we get by abstract- ing from the qualitative multiplicity of the world of experience, has properties which we must not ascribe to a " primordial fluid ". It is made up of places which are all " heres " and " theres," and the movements which take place in it have directions which are " ups " and " downs," " rights " and " lefts ". In a homogeneous " primordial fluid " there would be no places but only positions with reference to imaginary axes, no "up" and "down," no " right " or " left ". It is thus, as Dr. Ward has seen, apparently only another name for the abstract conceptual space of abstract
