The early short work of Thomas Aquinas Being and Essence argued very influentially that the essence or “whatness” of a thing can be understood independent of its existence, and therefore its existence must be something separate from its essence. Aquinas’ fellow student of Albert the Great, Dietrich of Freiberg (1250-1310), held on the contrary that there is no real distinction between being and essence, and that they are only distinct by their modes of signification, or ways in which they are said of things.
Aquinas points out that although essence is also called form, properly speaking the essence of non-simple beings or “composite substances” not only should not be identified with matter, but also should not be identified with form. The essence of a composite substance must include both the form and the matter. However, according to Aquinas, the composite of form and matter still has to separately be given being or existence by the Creator.
In contrast to common neo-Augustinian views that were more sharply dualistic than Augustine himself, Aquinas advanced a distinctive variant of hylomorphism, or the interdependence of form and matter. “Through the form, surely, which is the act of the matter, the matter is made a being in act and a certain kind of thing” (chapter 2).
This is a novel formulation using Aristotelian vocabulary in a somewhat Aristotelian style, and it has a kind of intrinsic appeal. The hylomorphic and implicitly anti-dualist sentiment is admirable. The suggestion that “form is the act of the matter” is appealing in its simplicity, but I don’t find Aristotle using act to explain the relation of form and matter.
Aristotle leaves the relation of form and matter happily underdetermined. He is more interested in ranges of variation of concrete things than in propositions with all abstract terms, however intriguing. That through the act of the matter, the matter is made a being in act is an intelligible and interesting claim, but I don’t think Aristotle would have said that either.
The notion of “act” in Aquinas is moreover not the same as it is in Aristotle. As Gwenaëlle Aubry notes, pure act in Aristotle is a final cause only, and explicitly not an efficient cause, whereas pure act in Aquinas is action, which is precisely and primarily an efficient cause. Aristotle’s first cause is the good that attracts beings; Aquinas’ God creates beings from nothing.
Aquinas goes on to give a novel account of being, famously arguing that in God — but only in God — essence and existence are indistinguishable. God for Aquinas — “I am who am” — is pure Being. His essence is pure existence. He has no other essence, and alone of all beings essentially exists. All other beings get their being or existence from God, and therefore their essence is really distinct from their existence.
Aquinas’ teaching of God as pure Being has to be understood as deliberately counterposed to the neoplatonic One beyond being. It is also incompatible with Plato’s Good that is more ancient and powerful than being.
Dietrich of Freiberg made the Aristotelian point that being is said in different ways for different kinds of things. He argued against Aquinas that the essence of a thing refers to everything about it, just as its being does. A human and a human being, he says, are exactly the same thing. Essence as such already involves being, and there is no being that is not the being of some essence. There is a nice little French edition L’Être et l’essence (1996), edited by Alain de Libera and Cyrille Michon, that has Latin and French texts of both Aquinas’ and Dietrich’s works by this title, with commentary.
Dietrich’s own theology was a highly original Aristotelianizing Christian neoplatonism. He tended to identify God with the neoplatonic One. He was known for his interest in natural science, and developed a detailed, accurate account of the optics of rainbows. According to de Libera and Michon, he spoke of a kind of natural providence. Inspired by Arabic Aristotelianism, he developed an elaborate cosmological-ethical theory of creative intellect. He argued that the human intellect does not just passively receive images of things, but has an active, creative component.
Dietrich was in Paris in the 1270s. Without a doubt he later became a leading light of the German Albertism that de Libera and others have associated both with the so-called “radical Aristotelianism” condemned in 1277 and with the rise of German mysticism. His slightly younger contemporary Meister Eckhart is known to have been influenced by his work.