Aquinas revolutionized a thousand years of Christian tradition by rejecting Plato in favor of Aristotle. Plato maintained that ultimate reality consists of essence, whereas Aristotle maintained that existence is primary. For Plato, the world around us that we perceive with our senses contains nothing except impermanent, ever-changing objects. Plato reasoned that for our observations of the world to count as true knowledge and not just as anecdotal evidence, our minds need to make a conceptual leap from individual instances of things to general ideas. He concluded that there must be something permanent that lies behind and unites individual existences, and he referred to this something as “essence.” According to Plato, existence, or the everyday world of objects such as tables, chairs, and dogs, is inherently inferior to essence. Early church thinkers saw in Plato’s ideas a parallel to their own division of the universe into the inherently imperfect, corrupt world of matter and everyday existence and the perfect and heavenly world of spirit.

Aquinas follows Aristotle in concluding that Plato’s theory is deficient, in part because it is unable to account for the origin of existence and in part because it is unacceptably dismissive of existence. Holy Scripture states that after each of the six days of Creation, God saw that the fruit of his day’s work was “good” or even “very good.” Furthermore, when Moses asks God how he should refer to him, God responds, “I am that I am,” thereby equating himself with being. In other words, God is pure existence or Being itself. Aquinas argues that man’s purpose consists exactly in developing himself toward Being, not in attempting to escape Being. In the traditional church view prior to Aquinas, the difference between God and his creatures was one of kind, as existence was something that in itself separated us from God. In Aquinas’s view, the difference between God and his creatures is one of degree, and we are separate from God insofar as we do not have as much existence as God. Prior to Aquinas, traditional church thought maintained that existence was the chief impediment to the realization of our spiritual destiny. Aquinas held that our spiritual destiny consists precisely in the enhancement of our existence.

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