Note 173 — Chapter I · THE LOGICAL CHARACTER OF PSYCHOLOGICAL SCIENCE

And they cannot be so described without the introduction of psychological ideas. Thus, e.g., in classifying a series of implements dating from different periods in the history of civilisation, so as to throw light on the evolution of some particular type of tool or machine, we have to take as our fundamentum divisionis the adequacy with which the different varieties accomplish the kind of work they were designed to perform, and are thus committed at once to the use of the psychological concepts of purpose and satisfaction.