The most influential theory of thought in the thirteenth century goes back to Aristotle and has its foremost medieval defender in Thomas Aquinas. It rests on viewing mental representations or intelligible species, as Aquinas calls them, as sameness in form. The explanation for why thoughts are about something, exhibit intentionality, or represent is that the form of the object thought about is in the mind of the thinker.
On Aquinas’ view, the mind is nothing before it thinks of something. The active intellect abstracts the intelligible form from the particular sensitive form in the internal senses and places it in the potential intellect. The form placed there by the active intellect hence actualizes the potential intellect. The thought is also always universal on this view since it is immaterial and matter is the individuating principle, according to Aquinas. The immaterial intelligible species in the potential intellect constitutes the thought.
There are many problems associated with this view of mental representation. A famous problem is: why do the daffodils outside my soul not represent my thought about the daffodils? The forms inside and outside my mind are the same, suggesting that mental representation is symmetrical. Aquinas has a famous answer to this problem, which is that the daffodils in the garden do not represent my thought because of the mode of the form’s presence in them. The forms in the daffodils are really present whereas in my mind the universal form is spiritually or intentionally present.
The distinction between forms being really or spiritually present is central to Aquinas’ physics and natural philosophy. A form may be present somewhere without literally making whatever substance it informs into something else. Colors in the air, for example, do not make the air really colored: we see colors in the objects around us but not in the intervening air, although they must be there spiritually if sensation is to be a causal process. This means, of course, that the air must also represent the color, which entails that intentionality is not a mark of the mental for Aquinas. The air is not in itself a mind (for discussion, see Pasnau 1997, Chap. 2).
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