Until quite recently I have tended to use “concept” as a synonym for Aquinas’ terms “intelligible form” and “intelligible species” in the intellect. Aquinas does not use the Latin conceptus as a synonym for intelligible forms as far as I’m aware, but to students and non-experts in medieval philosophy “intelligible form” sounds too foreign. “Concept” seemed like an acceptable, familiar alternative.
But I have been reading Aquinas’ Treatise on the Trinity in the Summa and there he does use conceptus as a term, saying that “the mind’s interior concept/conceived thing (conceptus) is called a word (verbum).” (ST I, q. 34, a. 1, c. See also I, q. 27, a. 1.) As the Word of God, the Second Person of the Trinity originates from/is conceived by/is spoken interiorly by the Father. And a word/concept is distinguished from an intelligible species (I, q. 34, a. 1, ad 2.) An intelligible species is not posterior by nature to the act of understanding. But the concept/word is; it proceeds from the intellect. Hence a concept is to be distinguished from an intelligible form.
This distinction helps to solve a problem with Aquinas’ and Aristotle’s epistemology. Both of them hold that we come to understand something when the very same substantial form that exists in the natural substances of our experience comes to inform our intellect as well. It exists in the intellect not as a “natural form” but as an “intelligible form,” which has “intentional existence” rather than “natural existence.” The form of dog, for instance, takes on a new mode of existing when it informs our intellect; rather than making our intellect a dog, as the form does in its natural existence, it instead makes our intellect to know the essence of dog. Aquinas does not mean that there is a representative in our mind standing in for the essence of dog; rather the essence of dog itself is there, in an intentional mode of existence. Hence there is no problem in Aquinas’ theory of trying to bridge the gap between the mind and the external world by verifying how much the representation in our mind is like the dog-essence out there in Fido. This would be impossible since we would have no independent access to the essence out there to verify the similarity/lack of similarity in the representation. According to Aquinas, the form in our intellect is the very same form as that in Fido, Rex, Chester, and Rover, and so there can be no question of similarity/lack of it.
This understanding, if plausible, solves the “content fallacy” objection to Aquinas’ argument for the immateriality of the intellect. For it is not that the mind has something that represents an immaterial, universal essence. In that case, the content/thing represented could be immaterial while the representation and its subject (the intellect) could be material. Rather, to know the universal and hence immaterial essence, the mind must itself be informed by an immaterial, universal essence. And such an immaterial universal cannot exist in matter. Thus the intellect is itself immaterial. (The form of dog, say, is the form of particular material things, and hence exists in matter in its natural existence; but in its intentional existence it becomes immaterial and thus universal in its scope, no longer restricted to a particular instance of its kind.)
But Aquinas’ and Aristotle’s position here can seem implausible. For how can we say that we have the very same form in our intellect as exists in the world outside when we find that we can misapprehend the essences of things, and we need to revise and reconstruct our concepts of them? As a friend said to me recently, Aquinas thought that fire was an elemental body, and that turned out to be wrong. Surely Aquinas had enough experience of fire to possess the kind of phantasm from which the intelligible form of fire could be abstracted.
But here is where I think the distinction between an intelligible form and a concept becomes quite helpful. (I wish Aquinas himself brought what he said in his discussion of the Trinity about words/concepts into his epistemological texts. Perhaps he did somewhere and I’m not aware of it.) Our concepts are things that we construct; they originate from our intellect once it is informed by intelligible species. These concepts express to oneself, interiorly, what one knows. The concepts, not the intelligible species, are the things we are readily conscious of. Intelligible species are things by which we know the essences of physical things, but we do not have any ready cognitive access to the intelligible species themselves. On the other hand, the concepts we conceive in our minds are readily accessible to us.
But just as one can express oneself poorly to other people, so too the concepts one forms can express to oneself one’s intelligible forms poorly. We can misconstruct our concepts, and they can be in need of reconstruction. Most people, however, once they have formed their concepts, cannot think outside those concepts and have a hard time understanding other ways of looking at things. One of the things that philosophy is good at is teaching a person how to get behind his concepts and grasp other conceptual frameworks (think Kuhnian paradigm shifts; compare MacIntyre also.) Once one does this, one can then evaluate which conceptual framework more accurately captures reality. This involves comparison to one’s experience, but because sense-experience does not itself present essences or universals, to evaluate competing conceptual frameworks one needs more than a comparison to sense-experience. One must compare the conceptual frameworks in question to the intelligible forms that one has sub-consciously within one. Do the concepts one has been using accurately capture what one knows about reality? If not, what concepts might capture them better? Aquinas’ concept of fire was in need of revision, even if he possessed the intelligible form of fire in his intellect. Precisely because one has these intelligible forms in one’s intellect, the process of choosing between competing conceptual frameworks (e.g., Kantianism and Aristotelianism) is not a hopeless task. It is possible to see which set of purported concepts and principles is more true.
But unlike our concepts, the Word of God expresses God and His knowledge perfectly. The one Word eternally expresses God Himself and everything. Praised be the Blessed Trinity!