At De Anima, bk. II, 12, 424a17, the First Teacher writes:
Universally speaking, we should bear in mind that every sense is receptive of the form of sensible objects without their matter, and in a sort of way in which wax receives the impression of a signet-ring without the iron or gold, for the wax receives the impression of the golden or bronze [ring] not qua gold or bronze. Similarly, a sense too is affected by a thing which has color or flavour or sound, [not insofar as what each is called as a particular thing], but insofar as it is coloured or flavoured or sounding and according to the [informing principle]. Now the primary sense organ is that in which such a [sense] power resides, and it [i.e., the organ] is the same [numerically as the sense power] but the essence of each of them is different; for that which senses [i.e., the organ] would be a certain [extended] magnitude. However, neither the essence of a sense nor the corresponding sensation is a [extended] magnitude; they are, respectively, the power [of the sense organ] and a certain form [in that organ]. [Emphasis mine]
In his Sentencia libri De Anima, lecture 24, the Doctor Angelicus comments on the above thus:
[…]First, then, [Aristotle] says that it must be maintained in general, as true of all the senses without exception, that the senses receive forms without matter, as wax receives the mark of a ring without the iron or gold. This, however, would seem to be common to all cases of passive reception; every passive thing receives from an agent in so far as the agent is active; and since the agent acts by its form, not its matter, every recipient as such receives form without matter. Which indeed is sensibly apparent; e.g. air does not receive matter from fire acting upon it, but a form. So it would seem not to be peculiar to sensation that it receives form without matter.
I answer that, while it is true that every recipient receives a form from an agent, there are different ways of receiving form. Form received in a patient from an agent sometimes has the same mode of existence in the recipient as in the agent; which occurs when the patient is disposed to the form in the same way as the agent. For whatever is received is received into the being of the recipient; so that, if the recipient is disposed as the agent is, the form comes to be in the recipient in the manner in which it exists in the agent. And in this case the form is not imparted without the matter. For although the numerically one and the same division of matter that is in the agent does not become the recipient’s, the latter becomes, in a way, the same as the material agent, inasmuch as it acquires a material disposition like that which was in the agent. And it is in this way that air receives the influence of fire, and any other passive thing in Nature the action that alters its natural quality.
Sometimes, however, the recipient receives the form into a mode of existence other than that which the form has in the agent; when, that is, the recipient’s material disposition to receive form does not resemble the material disposition in the agent. In these cases the form is taken into the recipient ‘without matter’, the recipient being assimilated to the agent in respect of form and not in respect of matter. And it is thus that a sense receives form without matter, the form having, in the sense, a different mode of being from that which it has in the object sensed. In the latter it has a material mode of being, but in the sense, a cognitional and spiritual mode.
Aristotle finds an apt example of this in the imprint of a seal on wax. The disposition of the wax to the image is not the same as that of the iron or gold to the image; hence wax, he says, takes a sign, i.e. a shape or image, of what is gold or bronze, but not precisely as gold or bronze. For the wax takes a likeness of the gold seal in respect of the image, but not in respect of the seal’s intrinsic disposition to be a gold seal. Likewise the sense is affected by the sense-object with a color or taste or flavour or sound, ‘not in respect of what each is called as a particular thing’, i.e. it is not affected by a coloured stone precisely as stone, or sweet honey precisely as honey, because in the sense there is no such disposition to the form as there is in these substances; but it is affected by them precisely as coloured, or tasty, or as having this or that ‘informing principle’ or form. For the sense is assimilated to the sensible object in point of form, not in point of the disposition of matter.
Next, at ‘Now the primary sense organ’, he concludes about the organ of sense. Since from his teaching that sense receives forms into cognition immaterially, which is true of the intellect also, one might be led to suppose that sense was an incorporeal faculty like the intellect, to preclude this error Aristotle assigns to sense an organ, observing that the ‘primary sensitive part’, i.e. organ of sense, is that in which a power of this sort resides, namely a capacity to receive forms without matter. For a sense organ, e.g. the eye, shares the same being with the faculty or power itself, though it differs in essence or definition, the faculty being as it were the form of the organ, as was said above. So he goes on to say ‘an extended magnitude’, i.e. a bodily organ, is what receives sensation’, i.e. is the subject of the sense-faculty, as matter is subject of form; and yet the magnitude and the sensitivity or sense differ by definition, the sense being a certain ratio, i.e. proportion and form and capacity, of the magnitude.