Over these definitions by point of view, one might quibble a little; for those who thus define psychology are not always consistent with themselves. In other passages of their writings they do not fail to oppose psychical to physiological phenomena, and they proclaim the irreducible heterogeneity of these two orders of phenomena and the impossibility of seeing in physics the producing cause of the moral. Ebbinghaus is certainly one of the modern writers who have most strongly insisted on this idea of opposition between the physiological and the psychical, and he is a convinced dualist. Now I do not very clearly understand in what the principle of heterogeneity can consist to a mind which admits, on the other hand, that psychology does not differ from the physical sciences by its content. However, I confine myself here to criticising the consequences and not the starting point. The definition of the psychical phenomenon by the point of view seems to me correct, although it has more concision than clearness; for it rests especially upon a material metaphor, and the expression "point of view" hardly applies except to the changes of perspective furnished by visible objects. It would be more exact to say that psychology specially studies certain objects of cognition, such as those which have the character of representations (reminiscences, ideas, concepts), the emotions, the volitions, and the reciprocal influences of these objects among themselves. It studies, then, a part of the material world, of that world which till now has been called psychological, because it does not come under the senses, and because it is subjective and inaccessible to others than ourselves; it studies the laws of those objects, which laws have been termed mental.[37] These laws are not recognised, popularly speaking, either in physics or in biology; they constitute for us a cognition apart from that of the natural world. Association by resemblance, for example, is a law of consciousness; it is a psychological law which has no application nor counterpart in the world of physics or biology. We may therefore sum up what has been said by the statement that psychology is the study of a certain number of laws, relations, and connections.