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Summa Theologica: The Nature and Limits of Human Knowledge

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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.

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