Thomas Aquinas (1224–1274) was very interested in the question, “What does it mean to be?” He sought to understand reality as a whole and tried to formulate explanations of all experience in terms of ultimate causes. About metaphysics in relation to its considerations of immaterial substances, he said, “Although this science considers these items, it does not think of each of them as its subject; its subject is simply being in general.” Taken literally, this claim about metaphysics describes it as transcendent of religion, because religious entities have being and their being is the subject of the most general philosophical study. Metaphysically, Aquinas determined that every being is distinct and undivided (unum), it has meaning (verum), and there is something good about it (bonum).

Aquinas distinguished between what a being is and that it is. What it is, is its essence, and that it is, is its esse. We can know the essences of things without considering their existence, but it requires an act of judgment to determine esse, that something is.