Page:Mind (Old Series) Volume 12.djvu/60

 PSYCHOLOGICAL PRINCIPLES, (ill.) 47 merits of empirical knowledge. 1 But where intercommuni- cation is out of the question, and where the physical life and conditions are widely different from our own, we are left to more or less probable conjecture. Till we have correlated the form of mind we do know to its nervous organisation, it seems hopeless to attempt inferences con- cerning the minds of the lower animals on the basis merely of what we know of their comparative anatomy. The psychologist who essays to treat mind evolutionally has to begin at the top of the chain and work downwards ; he can- not, like the biologist, begin at the bottom and work upwards. The problem for him is in large measure an inverse pro- blem and beset with many of the characteristic difficulties of such a method. His one chance of anything like scientific exactness lies in securing first of all an accurate and complete general analysis which shall tally, as far as the nature of the case admits, with what has been independently ascertained of the anatomy and physiology of the nervous system. And it is in this part of his work that he has much to learn from the modern physicist. It is a mistake to suppose that all the exactness of modern, physics is due to measurement, and to suppose accordingly that psychology can never be rendered exact till it becomes psycho- metry. In one important respect physics is exact even where concrete quantitative determinations may be impracticable; that is to say, the dimensions of a quantity may be known even when its numerical magnitude is not. All physical quantities that are not simply lengths, times or masses are expressible in terms of these fundamental units, and every equation that claims to have a physical meaning must involve only like dimensions of these units as far as it involves them at all. We cannot, e.g., equate so much momentum with so much energy any more than we can so much length with so much area. Any equality that is true of two physical quantities must obviously remain true whatever be the unit of measure- ment employed ; but then the dimensions must be the same, else a change of unit will unequally affect the numerical value of the two quantities. But further prolixity would be unpardonable in what is only meant to serve as an illustra- tion : it is time to turn to the point to be illustrated. A physicist never confounds velocity and acceleration, since they have different dimensions in time ; or energy and work, 1 It is a stupid confusion to represent this knowledge as ' subjective ' in the sense of being true only of a sui generis M or N ; as if there were no human kind.