(c) Relation to Subjective Activity. In perception we are relatively passive and receptive, because sensations are determined by a factor which is not psychical at all, — the stimulus. What the stimulus does for us in perception, we have to do for ourselves in ideation. Images are attended to only so far and so long as they connect themselves with the general direction of mental activity at the moment or arouse a new current of activity by bringing into play preexisting cognative tendencies. Sensations on the other hand tend by their sensational intensity to compel attention. If they are sufficiently intense they may forcibly divert attention from the most absorbing train of thought.

* Analytic Psychology, vol. ii., p. 185.

Percepts, so long as the stimulus persists on which they depend, display a steadiness which is absent in case of images. Images are maintained before consciousness purely by an effort of attention; when we are attending to a percept, sensational intensity due to the stimulus cooperates with our subjective activity, steadfastly sustaining it. Now attention is never perfectly fixed and continuous. It flags at intervals and constantly tends to pass from one point to another; it is probably subject to a regular rhythm of remission and concentration. Hence the peculiar unsteadiness of images even when we deliberately attempt to arrest and detain them. As Dr. Ward says, the image, in spite of our efforts to fix it, "varies continually in clearness and completeness, reminding one of nothing so much as of the illuminated devices made of gas jets common at fetes, when the wind sweeps across them. . . . There is not this perpetual flow and flicker in what we perceive."*

* Article "Psychology," p. 58.

Dr. Ward perhaps goes too far in attributing this "flow and flicker" to all mental imagery. The statistical evidence seems to show that some exceptionally gifted persons can maintain a visual image before their mental view without these fluctuations. But even in these cases the detention of the image costs a kind and degree of mental exertion which is not required in attending to percepts. The same contrast manifests itself in another way when we compare perceptual change and transition with the sequence of ideas. Ideas follow each other in accordance with purely psychological conditions; their sequence is determined by preformed associations together with the general trend of mental activity at the moment. The flow of ideas thus partakes of the nature of a continuous development of the attentionprocess. Changes in the content of perception, on the contrary, are only partially initiated by the changing direction of attention. They are determined to a very large extent by alterations in the nature of the stimulation affecting the organs of sense. So far as this is the case they bear the character not of a continuous development of conscious process, but of something which happens in consciousness. This character is most conspicuous when external changes suddenly introduce experiences for which the mind is unprepared, as when the chair we are sitting on unexpectedly gives way beneath us. But even when we are awaiting an event and are prepared to act appropriately when it comes, there is still a certain discontinuity or abruptness in the mode of its occurrence in consciousness as compared with the sequence of ideas in a train of thought. It is not a continuation of our own mental activity; it is something which happens to us, something which strikes upon the mind from without.

(d) Relation to Motor Activity. Inasmuch as percepts depend on external stimulation proceeding from surrounding things, they must vary with the spatial relations of the organism and its parts to environing conditions. Hence our perceptions vary with our movements. We can carry our ideas about with us; but if we turn our head away or close our eyes we can no longer see what we saw before. In particular the sensations we receive vary with the adaptation of our senseorgans. For most distinct vision we bring the eye into such a position that the rays from the object fall on the yellow spot; we accommodate the lens so that they form a distinct image on the retina, and so forth. The presence of these motor adjustments forms an important distinction between actual vision and visual imagery. The same holds good mutatis mutandis of the other senses.

It is true that there is also an adjustment to images, and that this adjustment consists in great part of a revival of the motor experiences which enter into actual perception. But the revival is easily distinguishable from actual movement. There is a difference of general attitude. In merely imaging "the attention feels as if drawn backward, towards the brain." The motor revival exists side by side with the sensations due to the actual state of the organism and its parts. We may be scanning a mental picture, and this may involve some ideal revival of the motor processes involved in actual vision. But at the same time we abstain from the corresponding active movements of the eye. The bodily eye may even be closed. Thus the motor revival is the more easily distinguished from actual movement, because the actual sensations of position and movement which we receive from the eye are incompatible with the movements which are ideally reproduced. The ideal movements appear therefore to occupy an inner circle. Extruded from the periphery they seem to take place within the head.