To explain the process of knowledge, Thomas Aquinas has recourse neither to the innate ideas of Platonism nor to the illumination of Augustine. Instead, he postulates a cognitive faculty naturally capable of acquiring knowledge of the object, in proportion to that faculty.
Agreeing with Aristotle, he admits that knowledge is obtained through two stages of operation, sensitive and intellective, which are intimately related to one another. The proper object of the sensitive faculty is the particular thing, the individual; the proper object of the intellect is the universal, the idea, the intelligible.
But the intellect does not attain any idea unless the material for that idea is presented to it by the senses: “Nihil est in intellectu quod prius non fuerit in sensu.” The two cognitive faculties, sense and intellect, are naturally capable of acquiring knowledge of their proper object, since both are in potency — the sense, toward the individual form; and the intellect, toward the form of the universal.
The obtaining of the universal presupposes that the sensible knowledge of the object which lies outside us comes through the impression of the form of the object upon the sensitive faculty. This is likened to the impression of the seal upon wax.
Upon this material impression the soul reacts according to its nature, that is, psychically, producing knowledge of that particular object whose form had been impressed upon the senses. Thus the faculty which was in potency is actuated with relation to that object, and knows and expresses within itself knowledge of that particular object.
But how is the passage made from sensitive cognition to that which is intellective? Or, rather, how is the individual form which is now offered by sensible cognition condensed into an idea and thus made the proportionate object of the intellect?
To understand the solution to the problem, it is necessary to recall the theory of Aristotle which Aquinas makes his own; that is, that the individual form is universal in potentia. It is the matter which makes the form individual. Hence if the form can be liberated from the individualizing matter, or dematerialized, it assumes the character of universality.
According to Thomas Aquinas, this is just what happens through the action of a special power of the intellect, i.e., the power by which the phantasm (sense image) is illuminated. Under the influence of this illumination, the form loses its materiality; that is, it becomes the essence or intelligible species (species intelligibilis). Thomas call this faculty the intellectus agens (agent intellect), and it is to be noted that for Thomas the “intellectus agens” is not, as the Averroists held, a separate intellect which is common to all men.
For Aquinas, the agent intellect is a special activity of the cognitive soul, and it is individual and immanent in every intellective soul. The “species intelligibilis” is then received by the intellect, which is called passive since it receives its proper object, and become intelligible in act.
Note that according to Aquinas the form, both intelligible and individual, is not that which the mind grasps or understands (this would reduce knowledge to mere phenomenalism), but is the means through which the mind understands the object (individual form) and the essence of the object (“forma intelligibilis”).
Knowledge thus has its foundation in reality, in the metaphysical.
Furthermore, since the cognitive faculty is in potency, when it becomes actuated, it becomes one with the form which actuates. Thus it may be said, in a certain sense, that the intellect is identified with the determined form which it knows.
For Aquinas all the data of sense knowledge and all intelligible things are essentially true. Truth consists in the equality of the intellect with its object, and such concordance is always found, both in sensitive cognition and in the idea. Error may exist in the judgment, since it can happen that a predicate may be attributed to a subject to which it does not really belong.
Besides the faculty of judgment, Aquinas also admits the faculty of discursive reasoning, which consists in the derivation of the knowledge of particulars from the universal. Deductive, syllogistic demonstration must be carried out according to the logical relationships which exist between two judgments. In this process consists the science which the human intellect can construct by itself, without recourse either to innate ideas or to any particular illumination.