St. Thomas Aquinas’ On Being and Essence

In St. Thomas Aquinas’ On Being and Essence, he devotes an entire chapter of his book discussing how essence is found in composite substances. “Form and matter are found in composite substances, as for example soul and body in man. But it cannot be said that either one of these alone is called the essence.’ Aquinas argues that in a composite substance, not only is the form but also matter in the essence of a thing. However, in Metaphysics, Aristotle says that essence is in the form, which acts upon matter. He writes, “The form or the thing as having form should be said to be thing, but matter by itself must never be said to be so.” Yet, Aristotle’s thesis poses a philosophical problem. If one supposes that Aristotle is correct, then how can one think of something without it necessitating its physical existence? This essay will first be an exposition of the passage found in Aquinas’ On Being and Essence. The second part of this essay will be an analysis of Aquinas’ thesis in relation to Aristotle’s. It will also address Aquinas’ solution to necessitating existence.

In Aristotle’s Metaphysics, he says the form is in essence, which then “informs” the matter. However, Aquinas argues that both form and matter are found in essence. Neither matter-alone nor form- alone can be called the essence, according to Aquinas. Since, essence is indicated by the definition of a thing, Aquinas asserts that the definition of “natural substances includes not only form but also matter.” Using a syllogism can properly articulate Aquinas’ argument:

Major Premise: Essence is signified by a thing’s definition

Minor Premise: Definitions of natural substances include form and matter.

Conclusion: Therefore, form and matter are in essence.

He us...

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