without the "matter" of the things in which those forms are exhibited. Thus the process of sense-perception is one in which the organ of sense is temporarily assimilated to the thing apprehended in respect of the particular quality cognised by that organ, but in respect of no other. According to Aristotle this process of "assimilation" always requires the presence of a "medium." If an object is in immediate contact with the eye we cannot see its colour; if it is too near the ear, we do not discern the note it gives out. Even in the case of touch and taste there is no immediate contact between the object perceived and the true organ of perception. For in touch the "flesh" is not the organ of apprehension but an integument surrounding it and capable of acting as an intermediary between it and things. Thus perception is always accomplished by a "motion" set up in the "medium" by the external object, and by the medium in our sense-organs. Aristotle thus contrives to bring correct apprehension by sense of the qualities of things under the formula of the "right mean" or "right proportion," which is better known from the use made of it in the philosopher’s theory of conduct. The colour of a surface, the pitch of the note given out by a vibrating string, &c., depend on, and vary with, certain forms or ratios "in" the surface or the vibrating string; our correct apprehension of the qualities depends on the reproduction of the _same_ ratios in our sense-organs, the establishment of the "right proportion" in _us_. That this "right proportion" may be reproduced in our own sense-organs it is necessary (1) that the medium should have none of the sensible qualities for the apprehension whereof it serves as medium, _e.g._ the medium in colour-perception must be colourless. If it had a colour of its own, the "motion" set up by the coloured bodies we apprehend would not be transmitted undistorted to our organs; we should see everything through a coloured haze. It is necessary for the same reason (2) that the percipient organ itself, when in a state of quiescence, should possess none of the qualities which can be induced in it by stimulation. The upshot of the whole theory is that the sense-organ is "potentially" what the sense-quality it apprehends is actually. Actual perceiving is just that special transition from the potential to the actual which results in making the organ for the time being _actually_ of the same quality as the object.