In question 84 of the prima pars, Thomas Aquinas attempts to reconcile his adoption of Aristotle’s epistemology contra Plato’s theory of innate intelligible species with his allegiance to the traditional Augustinian doctrine of divine illumination. His answer to this question was quite novel in its time, and considering the influence of his philosophy on successive generations, quite revolutionary. Robert Pasnau relates a bit of the substance of and the historical reaction to Aquinas’s particular theory of illumination:
Many of Aquinas’s early adversaries noticed only the negative aspect of Aquinas’s claims about divine illumination. Roger Marston (c. 1235-1303) spoke of those who, “drunk on the nectar of philosophy … twisted toward their own sense all of Augustine’s authoritative texts on the unchanging light and eternal rules” (De anima 3 ad 30 (p. 273)). Viewed only in the light of [Summa Theologiae I] 84.5, it is not clear why there should have been such hostility. The view that Aquinas here rejects is one that was universally rejected at the time: no one held that human beings in this life have the divine ideas as an object of cognition. Moreover, everyone agreed that divine illumination is not sufficient on its own, without the senses. Still, Aquinas would be controversial because he understands divine illumination to have occurred at the time when the soul was created. Others, in contrast, would argue for special illumination, an ongoing influence from above, constantly required for the intellect’s operation. From one perspective this introduces an important difference between Aquinas and his opponents. According to Henry of Ghent, for example
“It should be said unconditionally, therefore, that there is nothing a human being can have pure truth about by acquiring knowledge of it through purely natural means. Such truth can be had only through the divine light’s illumination” (Summa I.2 [f.8rM]).
Aquinas in contrast, is able to hold that
“Just as other natural active capacities, when connected to their passive counterparts suffice for their natural operations, so also the soul, which has within itself an active and a passive capacity, suffices for the perception of the truth” (InDT I.Ic).
It is not that Aquinas denies God’s influence, merely that he thinks this influence does its part at the outset, furnishing human beings with a sufficient capacity for thought on their own, without the need for any “new illumination added onto their natural illumination” (Ia2ae 109.Ic). It is this step toward naturalism that Aquinas’s opponents would find so objectionable. Thus Marston decries his opponents for holding that “we see all things in the first truth because we see in a light derived from that truth – namely, in the natural light of our mind, which is part of the soul” (De anima 3 resp. (pp. 252-53). (Pasnau, Thomas Aquinas on Human Nature, 305).
Pasnau notes that Aquinas’s epistemology is motivated by his goal to provide a justification for sense knowledge as an essential aspect of the hylomorphic nature of humanity. For Aquinas, the Platonists (those whom he knew primarily through Macrobius) had not sufficiently done this. For them the body does not aid but provides an obstacle to the intellect that must be removed. Rather, the active intellect depends upon the senses for its reception of phantasms which it uses in its crafting of intelligible species within the passive intellect. Nonetheless, this crafting is done by an innate power which “comes to the soul from the separated substances and especially from God as from its first source” (De Veritate., 10.6).