What, then, is this set of conditions which must be fulfilled? For one thing, the mental act must be directed upon something other than itself—something which comes to be regarded as a physical object. If, now, this act itself, as thus directed, is intro spectively scrutinised, it reveals itself not at all as an act of constructing the parts of that of which it becomes aware. Viewed from within, it invariably evinces itself as a process, not of manu facturing an object, but of differentiating the features of one. and of tracing connections which were not at first discerned. It evinces itself, that is to say, as an act of discriminating, and this, rather than, as Kant maintained, synthesising, would seem to be its essential nature.
In the situation described as "perception of an object" two concrete facts are, accordingly, implicated, the object on the one hand and the act or process of perceiving it on the other. And if, following Aristotle, we distinguish between the existence and essence or content, between the "that" and the "what," of any concrete fact, we may say that each of these concrete facts exhibits the two aspects of existence and content.
But, in view of the circumstance that in this situation the con scious subject is gradually discriminating the content of the ob ject, we need to distinguish what may be called the content appre hended both from the content of the object and from the con tent of the mental act itself. The content of the mental act is or becomes, partially at least, the awareness of qualities which are taken to be qualities of the object, and it would seem clear that the qualities in question must be distinguished from the aware ness of them. Furthermore, as distinguished from the content of the mental act, the content apprehended is that which is fre quently designated the "appearance" of the object to the per cipient. Now, again, this "appearance" can hardly be the object upon which the mental act is directed, because in order that it should arise at all it would seem essential that the object should be already there. Moreover, the sum of qualities of which the conscious subject will be aware at any moment will be different from the sum of qualities of which he will be aware at another moment, and either of these will be, at the best, but a fragment of the much larger sum of qualities which the object itself pos sesses. It follows, therefore, that the sum of apprehended fea tures (i.e., the content apprehended, or the "appearance" of the object) is distinguishable from the larger sum of features consti tuting the whole content of the object.
If this analysis be so far on the right lines, an important conse quence can at once be deduced,—namely, that the content appre hended or the "appearance" cannot be itself an existent. For it has evinced itself as only a selection from the features forming the content of the object, and ex hypothesi the content of any thing is not to be confused with its existence. So far from being there, as an existent entity, prior to the act of perception, and in some way calling forth that act, it only comes to be in virtue of the perceiving act being directed upon an existent object, and apart from the perceiving act it would have no "being" of any kind.
The main issue in regard to the nature of perception centres round the point just referred to. If the content apprehended be regarded as an existent—be it of the kind called an "idea" or a "presentation" or a "sensum" or what not—it assumes the posi tion of a tertium quid between the apprehending mind and the physical thing, and the old difficulties of the empirical theory as propounded by Locke recur. It is, indeed, difficult to see how, in that case, it can be said to be an "appearance" of a physical object at all. Moreover, if apprehended contents are existents, they are existents of a very peculiar kind. They cannot be de scribed as physical existents, for they do not, as such, act and react upon each other ; they do not obey the law of gravitation, or any other physical law; they are not modes of energy. And equally they cannot be described as mental existents, at all events not in the sense in which states of feeling or of cognising or of conation are so described.
Further, their mode of generation is even more peculiar. If they are generated by physical and physiological processes, it must be in a way totally different from that in which change in one physical thing is said to be the cause of change in another, and why physical existents acting on other physical existents should give rise to existents which are not physical is a mystery. On the other hand, if they are generated by reaction of the mind on stimulation, it is no less inexplicable how the mind, being of the nature the psychologist finds it to be, should give rise to an immense number of qualities altogether unlike in character the qualities it is known to possess. Meanwhile, then, let us assume that "appearances" or "apprehended contents" are not, as such, existents, and see how far on that assumption we can advance to a coherent view of the nature of knowledge.