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Here, St. Thomas reaches the peak of his ascent. From here, he will continue down, judging all things in light of this most proper attribution, i.e., "quod subsistit in Deo, est suum esse." (what subsists in God is His act of be)
To understand the profundity of this article, we must first consider the nature of esse. We have already considered a number of relations between the "passive" and "active" in the thought of St. Thomas.
We have looked at the relationship between "matter-form" and "suppositum-nature," but we have no looked at the relationship between "essence-esse" under the same light.
Most simply translate esse as "being." Yet, such a term is of common usage and can stunt the fruitfulness of our philosophical reflection unless we translate the meaning of the term in many words, i.e., by glossing the term, rather than by simply looking for an English equivalent.
By esse, we refer to the act of existence. Thus, all things that exist have an esse, else they wouldn't exist and thus be nothing.