§ 4. Sensation as Cognitive State distinguished from Sensation as Cognised Object.—We must distinguish the knowledge, of which sensations are the vehicle, from the knowledge which has for its object sensations themselves. It is true that without the sensations we can have no knowledge of them: but it is not true that whatever we know by means of sensations is knowledge of these sensations. We must distinguish between what a sense-experience means and what it is in its own intrinsic nature. The image thrown by an object on the retina of the eye decreases in magnitude as the distance of the object increases. This involves a corresponding difference in the visual sensation. When we deliberately fix our attention on the sensation and its phases, we may, with practice and by using appropriate means, notice this difference. We may become aware that a man entering a room and approaching us apparently increases in stature. But for the most part we ignore these variations in our experience. None the less, they fulfil a cognitive function. They help to determine our perception of the distance of the object seen. It is the business of the artist to attend to these and other differences in visual sensation, and reproduce them in his pictures. Only in this way is he enabled to effect an artistic illusion. He must reproduce differences of colour and of shading, etc., and differences due to the varying way in which objects in varying positions affect the eye. But for all this he needs a special training. He has to learn to notice what nobody notices in ordinary life. In ordinary life, people attend only to what the sense-experience practically means. The artist must acquire the power of attending to the intrinsic nature of the sense-experience itself.

Similarly, in psychology, we have to attend to sensations, as such: we have to examine their attributes as psychical states, and not merely their meaning as vehicles of knowledge. The two points of view only partially coincide. If we compare the colour red as a quality of a material object with the colour red as a quality of the corresponding sensation, we find that redness as immediately perceived is an attribute common to both. The difference lies in the different relations into which it enters in the two cases. As a quality of the thing, it is considered in relation to other qualities of the thing,—its shape, texture, flavour, odour, etc. As a psychical state, it is considered as a peculiar modification of the consciousness of the percipient, in relation to the general flow of his mental life. But this is not the only difference. When we are attending to redness as a sensation, we take cognisance of many characteristics which are usually ignored when we are only interested in it as a quality of material objects. The manifold variations which the colour of an object undergoes under varying phases of illumination are, to a large extent, ignored in ordinary perception, because they make no practical difference in the nature of the object as a physical thing. The colour is regarded as the same, and the illumination alone as varying. But for the psychologist, whose interest is in the sensation, and not in the physical object, these variations are of primary importance, and he, like the artist, must fix attention upon them.

Sensations, as such, therefore, are psychical states. These psychical states, as such, become objects only when we attend to them in an introspective way. Otherwise they are not themselves objects, but only constituents of the process by which objects are cognised.