In part 1 of the Summa, Aquinas begins his examination of the operation and limits of man’s intellect after discussing the soul and the union of body and soul. Questions 84, 85, and 86, each of which is subdivided into various Articles, address (1) the question of how the soul, when united with the body, understands corporeal things; (2) the mode and order of understanding; and (3) what our intellect knows in material things.

The soul knows bodies through the intellect by a knowledge that is immaterial, universal and necessary, although only God can understand all things. The cognitive soul has the potential to form principles of understanding and principles of sensation. Individual objects of our knowledge are not derived from Platonic forms but rather from the mind of God. Intellectual knowledge is formed by a conjunction of the passive senses and the active intellect. It is impossible for the intellect to understand anything without the mind forming phantasms, that is, mental images.

The intellect understands by abstracting from phantasms and thereby attains some knowledge of immaterial things. Our knowledge of things, though, is not the same as knowledge of our phantasms, for, if the two types of knowledge were the same, then the taste of honey, for example, could be either sweet or bitter, depending on the state of the perceiver. Rather, the phantasms are the means by which we come to understand things. Knowledge of individuals is prior to knowledge of universals.

The intellect is incapable of directly knowing individual things because it perceives them by means of phantasms. On the other hand, the intellect does perceive universals directly by means of abstraction. The intellect is potentially capable of understanding the concept of infinity insofar as it can form the idea of infinite succession, but it is actually incapable of comprehending infinity. Contingent things are known through sense experience and indirectly by the intellect, but necessary principles governing those contingent things are known only by the intellect. Although only God can know how the future will be in itself, we nevertheless can have some knowledge of the future insofar as we have knowledge of causes and effects.

Aquinas then proceeds to discuss additional questions pertaining to the soul, the production of the bodies of the first man and woman, human offspring, and man’s natural habitat. The Treatise on Divine Government concludes part 1 of the Summa.


Aquinas’s discussion of man’s capacity for knowledge occurs within the context of his discussion of man’s soul. This fact is significant, for it indicates that Aquinas believes that the intellect is not a capacity separate from the soul but a component of the soul itself. To have a soul is to have reason and intelligence. Aquinas thus accepts Aristotle’s notion that rationality is the essence of man, although Aquinas does not equate man’s entire essence with rationality.

Aquinas accepts the proposition that any knowledge that is to count as real knowledge must be universal, but he rejects Plato’s view that knowledge derives from a contemplation of ideas that exist latently and innately in the mind. Aquinas insists that the soul, which includes the intellect, would have no use for the body if, as Plato held, all knowledge were derived from the mind alone. Not only does Aquinas thereby affirm the necessity of the body and reject the notion that the body is an impediment to our acquisition of truth, he also rejects the doctrine of innate ideas. In other words, he contradicts Plato in asserting that there is nothing in the mind that was not first in the senses. At the same time, though, he says that the mind contributes to the acquisition of knowledge by forming “phantasms,” that is, mental images, that are ultimately derived from sense experience and by forming universal ideas and principles. Thus, sense experience provides the passive component of knowledge and the mind provides the active component of knowledge.

The mental images that we form are not universal knowledge itself. If we were to equate our mental images with universal knowledge, then we would be confronted with the problem of how to deal with the ideas that confused or even irrational people have. It would be absurd, for example, to say that honey is both sweet and bitter, but if all phantasms were to count as knowledge, we would fall into exactly such a radical subjectivism in which there was no objective standard of truth. Aquinas concludes that phantasms are indeed ultimately derived from individual things but require the abstraction that the intellect provides to rise to the level of being knowledge. This process of abstraction results in the formation of ideas of universals, that is, of ideas that define objects according to their essential qualities.

Aquinas arrives at the surprising notion that, although sense experience of a particular object is necessary to formulate both a mental image of that object and a universal concept that applies to that and all similar objects, knowledge of the particular material object, as that object is in itself, is impossible precisely because we have a mental image of it. It is true that we get to know the essence of the object through abstraction. Yet we do not, and indeed cannot, have knowledge of the object as a material object. Aquinas is thus saying that all knowledge worth the name “knowledge” is necessarily abstract.

This process of abstraction makes scientific knowledge, that is, knowledge of causes and effects, possible at all, and so we can have some knowledge of the future through scientific prediction. Nevertheless, the intellect has limits even with respect to abstract knowledge. We gain an abstract concept of infinity through the idea of infinitely adding numbers, for example, yet we are unable to comprehend an infinite series of numbers itself.