In the “Optics” the psychological remarks on Seeing and on Sense-Perception in general—are of special interest for us. Here he exerts himself to separate the individual Moments of the Perception, and to give prominence to the condition of Time as characterizing the whole process. Perception then is a compound process, arising out of (1) sensation, (2) comparison of several sensations or of the present sensation with the memory-image which has been gradually formed in the soul as a result of earlier sensations, and (3) recognition, in such fashion that we recognize the present percept as equivalent to the memory-image. Comparison and recognition are not activities of the Senses, which merely receive impressions passively, but they devolve upon the Understanding as the faculty of judgment. Ordinarily the whole process goes forward unconsciously or semi-consciously, and it is only through reflection that it is brought within our consciousness, and that the apparently simplex is separated into its component parts. The process of Perception is gone through very quickly. The more practice a man has in this respect, and the oftener a perception is repeated, the more firmly is the memory-image stamped upon the soul, and the more rapidly is recognition or perception effected. The cause of this is that the new sensation is supplemented by the image which is already present in the soul. One might thus be disposed to think that Perception was an instantaneous act, at least after long practice. That, however, would be erroneous, for not only is every sensation attended by a corresponding change localized in the sense-organ, which demands a certain time, but also, between the stimulation of the organ and the consciousness of the perception an interval of time must elapse, corresponding to the transmission of the stimulus for some distance along the nerves. That it needs time, for example, to perceive a colour, is proved by the rotating circle of colours, which shows us merely a mixed colour, because on account of the rapid movement we have no time to perceive the individual colours. Comparison and Recognition are, according to Ibn al-Haitham, the significant Mental Moments of Perception. On the other hand Sensation tallies with the Material; and the Sense experiencing the sensation exhibits a passive attitude. Properly all sensation is in itself a kind of discomfort, which ordinarily does not make itself felt, but which emerges into consciousness under very strong stimuli, for example, through too bright a light. A pleasurable character accrues only to the completed perception, that is to the recognition which lifts the material given in sensation, up to the mental form.