It must be admitted, for the senses in general, that each one is receptive of the perceptible forms of things without the matter, as wax takes the impress from a seal-ring, without the iron or gold of which the ring is made; —takes the device, that is, without the metal on which the device is inscribed. In like manner, the sense is impressed by each object having colour, or savour, or sound; not, however, after the appellation of the object but, according as it is of a certain quality, and in a given relation to the sense. It is the primal organ in which this faculty exists; and it is identical with the object perceived, although different from it in mode of being; for, otherwise, the percipient would be some kind of magnitude. But it cannot belong either to that percipient or to sensation to be magnitude, as they are rather a relation to, and a faculty for the perception of the qualities of each object. Thus, it is, from these reasons, made manifest why sentient impressions in excess destroy the sentient organs; for if the motion of the impression be stronger than that of the organ, then the relation which constitutes sensation is dissolved, as harmony and tone become discordant, when the chords are struck too forcibly.
But why do not plants feel, seeing that they also possess a living part, and are impressionable by tangible qualities? And that they are so impressionable is shewn in their being both cooled and heated; but the cause is that they have not that mediate faculty, nor any such principle as admits of their receiving only the forms of things; that along with forms they are affected by the matter also.
It may be questioned whether impressions can be made by odour upon what may be without smell, or by colour upon what may be without vision, and so for other qualities and senses. But if that which is smelt be odour, then odour, if it produce anything, must produce smell, and thus nothing without smell can be affected by odour, and the same holds good for the other senses; neither can beings which are sentient be affected, save in so far as they are sentient. All which is made evident in that neither light nor darkness, sound nor odour, can act upon bodies, although that which is present with them may, as air with thunder splits wood. But yet tangible and sapid qualities do act upon bodies; for, otherwise, by what could inanimate things be acted upon and changed? Shall it then be said that those other qualities also act upon bodies? But all bodies are not impressionable by odour and sound, and those which are so are indefinite and mobile, such as is the air; for the air gives out odour, as if it had been subject to impression. What then is smell but impression of some kind? But smelling is a sentient perception; and the air having been impressed by odour, becomes quickly sensible to us.