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 sense, (for we speak of one who hears and sees, in potentiality, as “one hearing and seeing,” although he may happen to be asleep, and we say the same of one who is actually employing those senses,) so may sensation be spoken of in two ways, as subsisting in potentiality and subsisting in activity. Let us, however, before proceeding further, observe that impression, motion, and action are for us equivalent terms—for motion is a kind of action, although an action which is incomplete, as has been elsewhere explained. Now, all things which are impressed and set in motion are so affected by something capable of making impression and existing in activity; so that impression is in one sense by like, and in another sense by unlike, as we have said—for the unlike is subject to impression, but, having been impressed, it is converted into like. A distinction, however, must be drawn between the terms potentiality and reality, for we are now going to employ them in an absolute sense—any individual whatever, then, may be learned, as we might speak of any man as learned, because man is among beings capable of learning and being learned; and so we speak of a man as learned, from his actually professing, at the time, grammatical or other knowledge.
Thus, each of these individuals is learned in potentiality, although in a different manner—the one is so because he is of a certain genus and peculiar matter; and the other, because he can when he will reflect upon his knowledge, provided there is no external impediment to his doing so. It is this one only, however, when actually reflecting upon his knowledge, being in activity, and fully acquainted with some one subject, as A. for instance, who is to be accounted learned in reality. Both those first men, in fact, are learned in potentiality; but the one is so from having been modified by learning, and undergone frequent changes from one habit to an opposite one; and the other is so from possessing sensibility or rudimentary learning, and being able, although in a different manner, to pass from inertia to activity.
But the term impression is not absolute in signification, as sometimes it implies a kind of destruction by a contrary, and sometimes it signifies rather preservation of something being in potentiality by something which is in reality and like, in the relation that potentiality bears to reality. Thus, the possession of knowledge implies the power of reflecting upon it, and this either is not change, being but an increase of knowledge and a step towards its completion, or it is change of a different kind. It is not correct, therefore, to say that an individual, when thinking, is undergoing change, any more than that a builder, when employed in building, is undergoing change; so that the process by which an individual passes, as to his thinking and reflecting faculties, from potentiality to reality, ought to have some other appellation than that of instruction. We may not then, as has been observed, say of the individual who, from being in potentiality, learns and receives knowledge from one who is in reality and able to teach, that he suffers impression, or else it must be admitted that there are two modes of change, one in privative dispositions, and another over habits and nature. The first change, however, of this kind in the sentient being comes from the parent at the moment of conception; as from that moment the being has, as it were, learning and sensibility. There is an analogy between the state of activity and reflexion just alluded to, but with this difference, that the impressions productive of activity, as the audible, the visible, and others, are all derived from without; and the cause of this is that sensation, in activity, is employed upon particulars, knowledge upon universals; and universals are, in some way, in the Vital Principle itself. The act of thinking, therefore, is dependant only upon the will of the individual, which is not the case with sentient perception, as for it there must of necessity be objects to be perceived; and this holds good, and for the same reason, with respect to the sciences which are engaged upon external objects, because all such objects are among particulars, and are external to the percipient. But an opportunity may hereafter present itself for the further elucidation of the subject. Let it, for the present, suffice to say, that the expression being in potentiality has not an absolute signification, for it may be understood of a boy as being qualified potentially to be a General, and also of an individual of suitable age for the office; and the term sensibility is subject to like modifications of meaning. But as the distinction between these two states of sensibility is without any special appellation, although it has been shewn that there is a distinction between them and what the distinction is, it has been found necessary to employ the terms impression and change, as if their signification were unequivocal; but, as has been said, the sentient principle is, when in potentiality, analogous to the external object when in reality.
The sentient principle, in fact, suffers impression when unlike; but, having been impressed, it is converted into like, and becomes the same as that by which the impression is made.