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.
But this does not deny the existence of differences between these groups of phenomena; neither does it assert that we can describe the characteristic features of one group in terms that belong to another group. Once a group of phenomena, biological, or chemical is there, we must have special formulae to describe them, otherwise there would be no need for these divisions. It is admitted that the earth was at one time destitute of life; it is also admitted that there are forms of life destitute of those features which we call mind. And, whatever be their mode of origin, once introduced they must be dealt with in special terms. Psychological facts must be expressed in terms of psychology, biological facts in terms of biology, and chemical facts in terms of chemistry. You may give the chemical and physical equivalent of a sunset. That is one aspect. You may also give the psychological explanation of the emotion of man on beholding it. That is another aspect. But you cannot express the psychological fact in terms of chemistry because it belongs to quite another category. A psychological fact, as such, is ultimate. So is a chemical or a biological fact. If by analysis you reduce the psychological fact to its chemical and biological equivalents, its character as a psychological fact is destroyed. That is the product of the synthesis, and to seek in analysis for what only exists in synthesis, is surely to altogether misunderstand the spirit of scientific method. The curious thing is that a mere layman should have to correct men of science on this matter.