3. PROBLEMS OF CONTEMPORARY EPISTEMOLOGY We may begin with a contention upon which stress was re peatedly laid by Lotze (1817-1881). As against the Hegelian con ception, Lotze regarded thought as a process of subjective activity, which neither passively mirrors the real nor necessarily corre sponds to the real. As an activity in rerum natura, it shares in the general traits that characterise real existence, and is adapted to real existence, but it reacts in its own way on the given, a way only capable of explanation by reference to its peculiar nature. So that, in constructing its picture of reality, it follows in part its own laws and in part is determined by the general characteristics of the given material.
Now, Lotze urges, that no effort of thinking in and for itself can render intelligible the significance either of concrete in dividual things or of change; and, in particular, change is that of which no thinking could ever inform us. It is only perception that can bring before us the fact of change. This contention rests upon a separation the legitimacy of which it is imperative to examine. Is it possible thus to sever these two mental processes, perceiving and thinking, without assigning to them so special a meaning as to deprive our general problem of more than half its significance? Doubtless, if we start by confining thought to those activities which operate on already given apprehended contents, what Lotze asserts of it is true. But what are we then to make of perception in which there is no element of thinking? What are we to make of an experience which is opposed to thought and which must there fore be presumed to be devoid of thought? In order to come to close quarters with the issue, it is necessary to determine the nature of an act of perception, and it has been with this problem that a great deal of recent research has been oc cupied. As the outcome of much labour and discussion there are certain propositions which most of those (except Bertrand Rus sell) who have devoted attention to the subject would accept.
These may be stated as follows :—(a) Perception as it takes place in ourselves is a complex act, involving much more than can be described as direct apprehension of sense-qualities. (b) So far as "sensations" are concerned, a distinction must be drawn be tween the act of sensing (sentire) and that which is sensed (sensum). (c) The sense-qualities or sensa of which we are aware in apprehending a physical object are never simply identical with the physical object itself or with any physical part of it and may not be identical with any qualities belonging to it. (d) What we know through sense-perception is based in the long run on the apprehension of sense-qualities and the perception of relations between apprehended sense-qualities. On the basis of these com monly accepted propositions, we can proceed to examine first the nature of our knowledge of physical objects by way of sense perception.
At the outset let us ask what is to be understood by the "given" which it is said perception simply accepts? What Lotze under stood by it is not doubtful. The "given" consisted, in his view, of impressions which the excitation of the nervous system called forth in the mind, and, until recently that has been the prevalent view. No one, indeed, doubts the fact that when a conscious being is (say) visually perceiving a coloured object, his visual organ has undergone stimulation, and that in consequence an influence of some sort has been conveyed to the cerebral centre with which the optic nerve is connected. But it may well be questioned whether the immediate result of that change is the production of a patch of colour either in the mind, or, in virtue of bodil) reaction, for the mind. On the contrary, what we do seem to be justified in asserting is that either concomitantly with or in con sequence of the cerebral change, there occurs a mental state or mode of activity in and through which, when a certain other set of conditions has been fulfilled, there ensues awareness of a definitely coloured object.