Objection 1. It would seem that the intelligible species are derived by the soul from some separate forms. For whatever is such by participation is caused by what is such essentially; for instance, that which is on fire is reduced to fire as the cause thereof. But the intellectual soul forasmuch as it is actually understanding, participates the thing understood: for, in a way, the intellect in act is the thing understood in act. Therefore what in itself and in its essence is understood in act, is the cause that the intellectual soul actually understands. Now that which in its essence is actually understood is a form existing without matter. Therefore the intelligible species, by which the soul understands, are caused by some separate forms.
Objection 2. Further, the intelligible is to the intellect, as the sensible is to the sense. But the sensible species which are in the senses, and by which we sense, are caused by the sensible object which exists actually outside the soul. Therefore the intelligible species, by which our intellect understands, are caused by some things actually intelligible, existing outside the soul. But these can be nothing else than forms separate from matter. Therefore the intelligible forms of our intellect are derived from some separate substances.
Objection 3. Further, whatever is in potentiality is reduced to act by something actual. If, therefore, our intellect, previously in potentiality, afterwards actually understands, this must needs be caused by some intellect which is always in act. But this is a separate intellect. Therefore the intelligible species, by which we actually understand, are caused by some separate substances.
On the contrary, If this were true we should not need the senses in order to understand. And this is proved to be false especially from the fact that if a man be wanting in a sense, he cannot have any knowledge of the sensibles corresponding to that sense.
I answer that, Some have held that the intelligible species of our intellect are derived from certain separate forms or substances. And this in two ways. For Plato, as we have said (1), held that the forms of sensible things subsist by themselves without matter; for instance, the form of a man which he called "per se" man, and the form or idea of a horse which is called "per se" horse, and so forth. He said therefore that these forms are participated both by our soul and by corporeal matter; by our soul, to the effect of knowledge thereof, and by corporeal matter to the effect of existence: so that, just as corporeal matter by participating the idea of a stone, becomes an individuating stone, so our intellect, by participating the idea of a stone, is made to understand a stone. Now participation of an idea takes place by some image of the idea in the participator, just as a model is participated by a copy. So just as he held that the sensible forms, which are in corporeal matter, are derived from the ideas as certain images thereof: so he held that the intelligible species of our intellect are images of the ideas, derived therefrom. And for this reason, as we have said above (Article 1), he referred sciences and definitions to those ideas.
But since it is contrary to the nature of sensible things that their forms should subsist without matter, as Aristotle proves in many ways (Metaph. vi), Avicenna (De Anima v) setting this opinion aside, held that the intelligible species of all sensible things, instead of subsisting in themselves without matter, pre-exist immaterially in the separate intellects: from the first of which, said he, such species are derived by a second, and so on to the last separate intellect which he called the "active intelligence," from which, according to him, intelligible species flow into our souls, and sensible species into corporeal matter. And so Avicenna agrees with Plato in this, that the intelligible species of our intellect are derived from certain separate forms; but these Plato held to subsist of themselves, while Avicenna placed them in the "active intelligence." They differ, too, in this respect, that Avicenna held that the intelligible species do not remain in our intellect after it has ceased actually to understand, and that it needs to turn (to the active intellect) in order to receive them anew. Consequently he does not hold that the soul has innate knowledge, as Plato, who held that the participated ideas remain immovably in the soul.
But in this opinion no sufficient reason can be assigned for the soul being united to the body. For it cannot be said that the intellectual soul is united to the body for the sake of the body: for neither is form for the sake of matter, nor is the mover for the sake of the moved, but rather the reverse. Especially does the body seem necessary to the intellectual soul, for the latter's proper operation which is to understand: since as to its being the soul does not depend on the body. But if the soul by its very nature had an inborn aptitude for receiving intelligible species through the influence of only certain separate principles, and were not to receive them from the senses, it would not need the body in order to understand: wherefore to no purpose would it be united to the body.
But if it be said that our soul needs the senses in order to understand, through being in some way awakened by them to the consideration of those things, the intelligible species of which it receives from the separate principles: even this seems an insufficient explanation. For this awakening does not seem necessary to the soul, except in as far as it is overcome by sluggishness, as the Platonists expressed it, and by forgetfulness, through its union with the body: and thus the senses would be of no use to the intellectual soul except for the purpose of removing the obstacle which the soul encounters through its union with the body. Consequently the reason of the union of the soul with the body still remains to be sought.
And if it be said with Avicenna, that the senses are necessary to the soul, because by them it is aroused to turn to the "active intelligence" from which it receives the species: neither is this a sufficient explanation. Because if it is natural for the soul to understand through species derived from the "active intelligence," it follows that at times the soul of an individual wanting in one of the senses can turn to the active intelligence, either from the inclination of its very nature, or through being roused by another sense, to the effect of receiving the intelligible species of which the corresponding sensible species are wanting. And thus a man born blind could have knowledge of colors; which is clearly untrue. We must therefore conclude that the intelligible species, by which our soul understands, are not derived from separate forms.
Reply to Objection 1. The intelligible species which are participated by our intellect are reduced, as to their first cause, to a first principle which is by its essence intelligible--namely, God. But they proceed from that principle by means of the sensible forms and material things, from which we gather knowledge, as Dionysius says (Div. Nom. vii).
Reply to Objection 2. Material things, as to the being which they have outside the soul, may be actually sensible, but not actually intelligible. Wherefore there is no comparison between sense and intellect.
Reply to Objection 3. Our passive intellect is reduced from potentiality to act by some being in act, that is, by the active intellect, which is a power of the soul, as we have said (79, 4); and not by a separate intelligence, as proximate cause, although perchance as remote cause.
Source: Summa Theologica (New Advent)