Before we go too far I would like to stop and define a couple important terms that will be used through out these posts. I will be as complete as possible, though I may end up having to define some terms in later posts.
By existent I mean an actually existing entity, a being. Now, being as being is the subject matter of Metaphysics. The natural sciences, however, do not study being as being, but qualified being. “All sciences are concerned with beings, but they do not consider all beings, but only a particular type of being, such as plants, minerals, or quantified being.” (Henry J. Koren, An Introduction to the Science of Metaphysics (1955; repr., St. Louis, MO: B. Herder Book Co., 1957), 7.) As Robert Bolton says, “At the beginning of Met. Γ Aristotle says of the special (non-universal) sciences that they do not concern themselves with any universal study of what is qua being but rather they each simply 'cut off a part of what is and study what happens to this' (1003a23-25).” (Robert Bolton, “Science and the Science of Substance in Aristotle’s Metaphysics Z,” in Form, Matter, and Mixture in Aristotle, ed. Frank A. Lewis and Robert Bolton (Oxford: Blackwell Publishers, 1996), 233-4.) Therefore, when we look at the existent we are looking at the aspect of being, which is the proper subject of the particular science.
In discussing being, it is necessary, at least briefly, to note the different ways in which Aquinas talks about being. In his glossary of terms W. Norris Clarke explains three uses of the word being. “Being = that which is. When used without qualification = a real being = that which actually exists with its own act of existence outside of an idea. When specified as a mental being = that which is present not by its own act of existence but only inside an idea; its being is its to-be-thought-about: numbers, possible, abstractions as abstract, hypotheses, etc.” (W. Norris Clarke, The One and the Many: A Contemporary Thomistic Metaphysics (2001; repr., Notre Dame, IN: University of Notre Dame, 2007), 316. There are two further distinctions to be made. There is what Aquinas calls ens per se, being in itself, and ens per accidens, which could be described as an accidental being, in the sense that it is not truly a being, its existence in our thoughts is due to our attaching a predicate to a subject, such as “fire man”. According to McCabe, it is “The combination of a substance and an accident.” (Herbert McCabe, On Aquinas, ed. Brian Davies (London : Continuum, 2008), 28.) According to Owens, “Being per accidens, because it lacks necessity and definiteness, cannot be learned or taught. It is as it were only a name, and seems something akin to non-Being.” (Joseph Owens, The Doctrine of Being in the Aristotelian Metaphysics, 2nd ed. (Toronto, ON: Pontifical Institute of Mediaeval Studies, 1963), 308.)
Ens per accidens is different from an accident, which seen by itself cannot be without a subject, we say that an accident has being in as much as it is found to be in – inest – a subject. (McCabe, 28.) Though Aquinas does not want to say that Ens per se is the definition of substance, (John F. Wippel, The Metaphysical Thought of Thomas Aquinas: From Finite Being to Uncreated Being (Washington D. C.: Catholic University of America Press, 2000), 234.) we may be permitted to say that ens per se is, at least, a subject in which is found an accident (Wippel says that “An accident is rather a ‘thing to which it belongs to be in something else.’” (Ibid., 234.)); and, if there is a word that describes the combination of subject and accident, this would be called ens per accidens. With this background we can understand that when we talk about an existent we are talking about an existing thing, a being, something in which an accident can be.