In physiological psychology the attempt is made to parallel so-called psychical phenomena with physiological processes. This chapter aims to make some allied suggestions with reference to Comparative Psychology. Professor William James refers to the whole nervous system, including the hemispheres, as an organized set of paths. A comparative psychology would result in which the corresponding phenomena would always be present. If comparative psychology means the comparison of the psychical phenomena of man with the same in the lower animals, no such presence of corresponding elements can be assumed. If there is to be a comparative science at all, there must be comparable terms, and the comparison must confine itself so far as it can hope to be scientific to such terms. The chapter illustrates the value of action by the mental image. The working image is one that arises through the conflict between the tendencies to action and the inhibition of these tendencies by the situation itself.