CHAPTER V

SENSITIVITY

POTENCY AND ACT IN SENSATION

             THESE questions being settled, let us speak of sensation in general. As has been said, sensation occurs in a being moved and acted upon; for it appears to be a kind of alteration. Some say, 'like is acted on by like'. How far this is possible or impossible has been stated in our general discussion of activity and passivity.§§ 350-1

             It may be asked why there is no sensation of the senses themselves; and why they do not produce sensation without something extraneous, seeing that they contain within themselves fire and earth and the other elements that give rise to sensation, either of themselves or through their accidental qualities. It becomes evident that the sensitive power is not an actuality, but is only potential; which explains why it does not sense [without an exterior object] as the combustible does not burn of itself without something to make it burn. Otherwise it would burn itself, and not need a fire already alight.§ 352-4

             But as we speak of sensing in two ways, (for we say that one who sees and hears in potency sees and hears, even when he happens to be asleep; and also that one does so actually) so we may speak of 'sense' in two ways,--as in potency and as in act. Likewise, to perceive is both potential and actual.§ 355

             To start with then, let us speak as if being acted upon and moved were the same as action and moving. For movement is a kind of activity, though imperfect, as has been stated elsewhere.§ 356

             All things are moved and affected by an agent, or something in act. Hence it is, that a thing is affected both by its similar and also by its dissimilar, as we have said. What is being affected is dissimilar: what has been affected is similar.§ 357