(A section from the diss. Presumes some things said earlier which are not posted. Some details added which didn’t fi in the original.)
In the eighth question of de potentia, St. Thomas argues that:
The conception [of the intellect] is considered as the term of an action, and as something constituted by it. For the intellect forms the definition of a thing by its action, either an affirmative proposition or a negative. But this conception of the intellect in us is properly called “a word” (verbum)
This word is understood in this manner:
The intellect understands something in two ways: in one way formally, and in this way it understands the intelligible species by which it is in act; in another way as an instrument which it uses to understand something, and in this way the intellect understands by the by the word.
Notice that the word is not what is understood, but that by which something is understood. But how far are we to press this opposition? St. Thomas pushes the opposition as far as it can go: the interior word or concept is, in its very nature, a “by which” something is known as opposed to “that which” is known. A likeness might be helpful: if something is outside our room, we can use two different instruments or means to know it; we can either have someone take a picture of it and bring it into the room, or we can put a window in the room so that we can look at the thing itself. In the case of the picture, what is understood (the quod of knowledge) is inside the room. In the case of the window, what is understood (the quod) is first outside the room, and we next reflect (if at all) about the properties of the window. For St. Thomas, the interior word is like the window and not the picture. If it were like the picture, we would properly and first know only things within the soul, but:
[I]t would follow that all science would not be of things outside the soul, but only of the intelligible species in the soul; as according to the Platonists all sciences are of ideas, which they posit as being in the intellect in act. Second, because it would follow from this the error of the ancients, saying that everything which appears is true and thus contradictories would be true at the same time.
If we must imagine the predicated universal, therefore, we should imagine it as a window and not a picture. There is an immediate objection to this, however, for in the very same article, St. Thomas says:
The known is in the knower through its similitude. And “the known in act is the knower in act” is said this way, inasmuch as the similitude of the thing known is the form of the intellect, just as the similitude of the sensible thing is the form of the sense in act. So it does not follow that the abstract intelligible species might be that which is understood in act, but that it be its similitude.
Clearly, St. Thomas is stressing that the similitude of the thing is in the intellect. Here again there is a familiar difficulty, for the imagination spontaneously offers us the idea of a picture or photograph to explain what is going on: if the mind has similitudes, it has pictures of things. But this image must be explicitly negated for several reasons. First, it is not in line with the clear argument in the body of the article. More importantly, however, to see similitudes as pictures is not in keeping with St. Thomas’s use of the word “similitude”. Consider this passage from the commentary on Peryermeneias which speaks to exactly the sorts of similitudes in question:
Letters are signs of words, and words of the undergoings [of the soul] which do not have the notion of similitude but only the notion of institution, as with many other signs: as a trumpet is a sign of war. In the undergoings of the soul there needs to be the notion of similitude for the expression of things, because they naturally designate them, not by institution.
In other words, St. Thomas does not use the word “similitude” so as to speak of an image of something as opposed to the reality of it, but to speak of a natural union between knowledge and the thing known as opposed to an instituted union, that is, a union that comes about by an act of the will. When St. Thomas speaks of the similitudes of the mind he wants to convey the idea of them coming to be by generation as opposed to art. Generation conveys the idea of a single species for the knowledge and the thing known, which cannot be said of the picture and the thing pictured. The image that we spontaneously form of the “similitude” of the intellect must be negated and replaced with the intelligible notion of a union in species between knowledge and the thing known. If we must form an image of the relation of the thing known to the universal, it is better to see it not as “something in the real world making a picture of itself in us”, but as an act of perfect generation, whereby a father (the external world) begets a son (the concept) that is perfectly like him in every way, so much so that the son gives us an immediate knowledge of the father through himself, the way an object is immediately seen through a window. The image is piecemeal, however, and has as many important negations as affirmations.
Worst of all, this image fails to take into account the role of the agent intellect, which empowers the world to form a properly intelligible image in us. So considered, the father (agent intellect) empowers the son (the world as illuminated) such that concept might come forth, making both the world known, and the intellect through reflection on the world.
 Questiones disputatae de potentia, q. 8 a. 1 co. “conceptio consideratur ut terminus actionis, et quasi quoddam per ipsam constitutum. Intellectus enim sua actione format rei definitionem, vel etiam propositionem affirmativam seu negativam. Haec autem conceptio intellectus in nobis proprie verbum dicitur”
 Quaestiones de quolibet V, q. 5 a. 2 ad 1 “intellectus intelligit aliquid dupliciter: uno modo formaliter, et sic intelligit specie intelligibili qua fit in actu; alio modo sicut instrumento quo utitur ad aliud intelligendum: et hoc modo intellectus verbo intelligit.”
 Summa theologiae I q. 85 a. 2 co along with the first part of the elided sentence:“[Si igitur ea quae intelligimus essent solum species quae sunt in anima, sequeretur] quod scientiae omnes non essent de rebus quae sunt extra animam, sed solum de speciebus intelligibilibus quae sunt in anima; sicut secundum Platonicos omnes scientiae sunt de ideis, quas ponebant esse intellecta in actu. Secundo, quia sequeretur error antiquorum dicentium quod omne quod videtur est verum; et sic quod contradictoriae essent simul verae.”
 Summa theologiae I q. 85 ad 1 “intellectum est in intelligente per suam similitudinem. Et per hunc modum dicitur quod intellectum in actu est intellectus in actu, inquantum similitudo rei intellectae est forma intellectus; sicut similitudo rei sensibilis est forma sensus in actu. Unde non sequitur quod species intelligibilis abstracta sit id quod actu intelligitur, sed quod sit similitudo eius.”
 “Undergoings” translates “passiones”. A transliteration would use the word “passions”, but English does not use “the passions of the soul” to express what St. Thomas is talking about here. He means to speak of the soul receiving or undergoing some influence from things. There is certainly no reference to the emotional life being made. “That which the soul is undergoing from things” might have more captured the sense.
 Expositio libri Peryermeneias, lib. 1 l. 2 n. 9 “Litterae autem ita sunt signa vocum, et voces passionum, quod non attenditur ibi aliqua ratio similitudinis, sed sola ratio institutionis, sicut et in multis aliis signis: ut tuba est signum belli. In passionibus autem animae oportet attendi rationem similitudinis ad exprimendas res, quia naturaliter eas designant, non ex institutione.”
 St. Thomas frequently speaks of the similitude that proceeds from generation.XXXXXX