Perhaps a word or two more on this topic is advisable. If it is permissible to arrange natural phenomena in a serial order, we may place them in succession as physical, chemical, biological, and psychological. But these names represent no more than descriptions of certain features that are to the group common, otherwise the grouping would be useless and impossible. And it is part of the business of science to frame "laws"--descriptions--of phenomena such as will enable us to express their characteristic features in a brief formula. It is, therefore, quite true to say that you cannot express vital phenomena in terms of physics or chemistry. And no materialist who took the trouble to understand materialism, instead of taking a statement of what it is from an anti-materialist, ever thought otherwise. _Each specific group of phenomena can only be covered by laws that belong to that group, and which were framed for that express purpose._ A psychological fact can no more be expressed in terms of chemistry than a physical fact can be expressed in terms of biology. These truths are as plain to the mechanist as they are to the vitalist. Mental life, the scientific categories, are real to all; the only question at issue is that of their origin. To explain is to make intelligible, and in that sense all scientific explanation consists in the establishing of equivalents. When we say that A, B, C are the factors of D, we have asserted D is the equivalent of A, B, C--plus, of course, all that results from the combination of the factors. When we say that we have explained the formation of water by showing it to be the product of H.2.O. we have shown that whether we say "water" or use the chemical formula we are making identical statements. If we are working out a problem in dynamics we meet with exactly the same principle. We must prove that the resultant accounts for all the forces in operation at the time. Now, all that the mechanist claims is that it is extremely probable that one day the scientist will be able to work out the exact physico-chemical conditions that are the equivalents of biological phenomena, and, in turn, the physico-chemical-biological conditions that are the equivalents of psychological phenomena. Very considerable progress has already been made in this direction, and, as Sir Oliver Lodge says, there are probably very few scientific men who would deny the likelihood of this being done.