All animals whose conscious processes rise above the level of the sensation-reflex must have some kind of apprehension of physical reality. It may be very rudimentary, and it may differ very much in its specific nature from our own perception of the external world. But there must exist for them some difference between experience so far as it merely involves their own changing states, and experience so far as it involves the presence and operation of external objects. The very conditions of their existence, from a purely biological point of view, render necessary a different mode of behaviour, according as the processes occurring in their nervous system are or are not due to the present operation of agencies external to their own organism.

I say external to their organism. The Self at this stage in the development of mental life is an embodied Self. The distinction between the pure stream of conscious experience and the body with which it is connected is a late product of psychical evolution. But there is a subtlety involved here which must be carefully noted. The body is directly identified with the Self only in so far as it is the instrument of senseperception. But one part of the body may be perceived by another part; the eye may look at the hand; in this case the hand as seen belongs pro tanto to the Not-Self; the eye, as instrument of perception, to the Self.

There are, as a matter of fact, two groups of experiences originating in different ways which we from our own point of view can clearly distinguish in the case of any animal which has to maintain its existence by perceptual adaptation to its environment. On the one hand, there are all kinds of organic sensations and appetites and active impulses and emotions which are relatively independent of external impressions on the organism. Their specific nature does not correspond in any definite manner with the specific nature of external agencies affecting the senses, and they may originate quite apart from the operation of such agencies. On the other hand, there is the stream of special sensations produced by external conditions, and varying from moment to moment as these conditions vary.

We from our point of view can easily draw this distinction. The problem for the psychologist is to inquire how the distinction manifests itself in the experiences of the percipient subject. The essence of the answer lies in the different relation of the two groups of experiences to motor activity. The sensations which vary in a specific or definite manner with the operation of external agencies, also vary continually with the movements of the animal itself. Visual sensations alter as the eye is moved; tactual sensations as the hand is moved.

The animal's movements alter its spatial relations to the things which surround it; and in this way determine, to a large extent, the nature and intensity of the impressions which it receives from its environment. But experiences occurring independently of the present operation of any external stimulus are unaffected by the changing position of the organism and its parts in relation to surrounding objects. The hungry animal carries its hunger about with it, and the wounded animal carries its wound about with it.

Our first result, then, is that the presentation of external objects arises in connexion with those experiences which vary with the changing position of the organism and its parts. It arises in connexion with experiences which are dependent on motor activity. But this result is not final. It only serves to bring us to the threshold of our inquiry. For it turns out on closer examination that in so far as an experience is merely dependent on motor activity it is not a presentation of an external object. If I walk towards an object, the visual sensation which it produces changes; it changes precisely as it would change if I had remained still and, instead of my moving, the object itself had moved towards me or had increased in size. But the change being produced by my own movement the object itself is not apprehended by me as moving or becoming larger. Similarly, if I get up from my chair I do not apprehend this as a movement on the part of the chair, but the case is different if the chair gives way under me. An animal, if it acted as if changes purely due to its own changing position were due to change in things themselves, would inevitably perish. So far as they depend merely on the changing positions and movements of the organism, they do not correspond to external conditions, and cannot therefore determine actions effectively adapted to these conditions. Just so far as they are due to the animal itself and not to its environment, they must be useless as determinants of the actual course of practical activity. There is a libellous story about the ostrich burying its head in the sand on the approach of danger, and resting satisfied with this sage precaution. To behave in this manner would be to behave as if the mere disappearance of an object from sight were equivalent to its actual removal. This is so only in case the conditions are such that, if it were present, it would be seen. The closure of the eyes, or the burying of the head in the sand, makes no difference in the external conditions; the consequent discontinuance of impressions arising from the dangerous object is merely a selfinitiated change without practical significance.