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I am required to do a term paper on the ontology of Thomas Aquinas. My question may seem rather trivial, but it would be very helpful if you would respond. I have tried defining the term ontology, in several books, however, it seems that I get a different definition for the term in each one, depending upon when the term was defined. If you could point me in the right direction ( a simplistic definition, and perhaps links) concerning the " Ontology" of Thomas Aquinas, it would be greatly appreciated. I am required to do between 10-20 pages, so any information will be useful. Thank you,


The term "ontology" as I understand it was coined in something like the 17th or 18th century. It comes from "ontos", the Greek word for being, and so literally means the study of being. I imagine you may have discovered this much already. Some philosophers have used ontology to designate a part of metaphysics as opposed to the philosophical study of God, for example. In this case, ontology might be like general metaphysics, the study of the metaphysical features of everything except God.

It is difficult to say from the little you have told me what your professor considers to be covered by the term "ontology of Aquinas." For Aquinas, metaphysics is the study of things in the most basic or most general way. In the Prologue to his Commentary on the Metaphysics of Aristotle, he delineates three branches of metaphysics: (1) the study of being as being, or of common being, (what all beings have in common in virtue of which they are beings) which also is called metaphysics (it is opposed to physics which studies being as changing); (2) the study of the principle(s) of the being of things, which is called First Philosophy (this branch studies the cause(s) of beings); (3) the study of things which can be known in no other way than as beings (since they are non-material, e.g. the nature of God and of angels), which is called philosophical Theology. For Aquinas, (1) is the primary sense of metaphysics since the other two are defined in relation to it.

If your professor has assigned for you the task of identifying and explaining Aquinas' metaphysics, he or she has given you quite a tall order, since entire books, and the life-times of scholars, have been occupied with this topic.

In An Elementary Christian Metaphysics by Joseph Owens (on pages 7-8) discusses the term "ontology" as used by Christian Wolff for the most general branch of metaphysics. He then says that this notion influences how contemporary philosophers treat Thomistic Metaphysics. "The viewpoint of an ontology, that is, of a general metaphysics distinct from philosophical theology, persists however in most Neoscholastic circles."(p. 8) I think that it is safe to assume, unless there is further elaboration from your professor, that he means the "ontology of Aquinas" to refer to the general systematic study of things according to their most general metaphysical principles; ontology is the study anything and everything as beings. Especially for Owens (who, by the way, is a follower of Etienne Gilson (author of many books on the subject)) it is this ASPECT, "as beings," that characterizes the study of metaphysics. ECM is Owens attempt to explain how we come to understand things "as beings," and what we can learn about them in their being. Section I of EMC, then, especially pertains to your topic. Basically, Aquinas' metaphysics tries to show that every created thing has two intrinsic principles of its being, its essence (what it is - which is known in a definition) and its act of existence (whereby the thing is a real thing).

A paper on the ontology of Aquinas should try to explicate what he means by being-in-general or being-as-being, why he thinks this needs to be explained by the principles of essence and act of existence, what these principles are, and how (if at all) these principles apply to God (and to the extent that they don't, why they don't). The work of Aquinas that is most important to Owens on this topic is On Being and Essence, especially Chapter 4. There is a sort of older translation of this work by Armand Maurer, and a newer one by Joseph Bobick; both have helpful notes and commentaries.

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