Let us now proceed, as those subjects have been scrutinized, to speak upon sensation in its widest acceptation.
Sensation is the combined result, as has been said, of a motion and an impression, for it seems to be some kind of change; and some writers maintain that it is only like which is impressionable by like, but we have already, in our treatises “upon action and impression,” shewn how far the opinion is or is not tenable. But it is difficult to understand why there is no sensation from the senses of themselves, that is, why, without the presence of external objects, the senses do not give out sensation, although fire, earth, and the other elements, from which or the accidents of which sensation is derived, are present in them. It is evident that it is because the sensibility is not in a state of activity, but is only in potentiality; and, therefore, that it is with it as with a combustible material, which alone, without something on fire, does not burn; for otherwise it might set fire to itself, and would stand in no need of fire, in reality, for the purpose. Since we speak of sentient perception in a two-fold
