So far in this series, we have covered two aspects of Thomistic metaphysics: first, being qua being as the subject of this science ; second, act and potency as intrinsic principles of being.  With these two aspects in view, we’ll move on in this article to the esse-essentia distinction, as found in Aquinas’ On Being and Essence.  That is, the distinction between existence and essence, as intrinsic principles, in a particular being. In this treatise, Aquinas investigates the meaning of the terms ‘being’ and ‘essence’ as it relates to various kinds of existents:
Since being and essence are the things first conceived of by the intellect, as Avicenna says at the beginning of his Metaphysics 1.6, in order to avoid errors
arising from ignorance about these two things, we should make plain the
difficulties surrounding them by explaining what the terms being and essence
each signify and by showing how each may be found in various things and
how each is related to the logical intentions of genus, species, and difference.
In the first chapter, Aquinas distinguishes between two meanings of the term ‘being,’ as per Aristotle : the first signifies that which is divided into the ten categories—substance, quantity, quality, relatives, somewhere, sometime, being in a position, having, acting, and being acted upon —while the second signifies the truth of propositions. The former mode of signification necessarily posits something in reality (i.e., an existent of some sort) while the latter mode does not necessarily posit anything in reality but rather refers to “anything…about which an affirmative proposition can be formed.”  ‘Being,’ then, is predicated more properly according to the first mode of signification rather than the second, since ‘being’ (Lt. ens) means ‘to be,’ with the assumption of meaning to-be-in-reality. ‘Essence’ originates from the first signification of ‘being’, since this “is what signifies the essence of a thing.”  Now ‘essence’ here “signifies something common to all natures through which the various beings are placed in the various genera and species.”  Aquinas also mentions the other terms generally equitable with ‘essence,’ such as quiddity (referring to the whatness of a thing), form, and nature. Since the proper subject of being is substances, it follows that “essence…is properly and truly in substances and is in accidents only in a certain way and in a certain sense.”  Then an essence exists when it is in a substance and a substance is of a particular sort when it contains an essence. This sets the stage for Aquinas to demonstrate the distinction between esse and essentia—that is, between the intrinsic principles of existence and essence. Aquinas’ formulation of this distinction comes in chapter four:
Whatever is not in the concept of the essence or quiddity comes from outside the essence and makes a composition with the essence, because no essence
can be understood without the things that are its parts. But every essence or
quiddity can be understood without understanding anything about its
existence: I can understand what a man is or what a phoenix is, and
nevertheless not know whether either has existence in reality. Therefore, it is
clear that existence is something other than the essence or quiddity, unless
perhaps there is something whose quiddity is its very own existence, and, if
so, this thing must be one and primary. 
Aquinas’ distinction arises from a consideration of addition to the concept of an essence. If the concept of an essence is added to extrinsically (that is, from outside the concept of the essence) then a composition is formed between the concepts of the essence and whatever is added to it. This is because in such cases an essence cannot be understood without reference to its parts. Take the Aristotelian definition of man as ‘rational animal.’ This is what is called a composite concept, since animal is not contained within the concept of rational, neither is rational per se contained within the concept of animal. Thus, in order to understand the human essence, one must have reference to the composition of animal with rationality. This is why Aquinas says “no essence can be understood without the things that are its parts.” However, it seems to be that an essence can be understood without any reference to its actual existence or not. Aquinas gives the example of a man and a phoenix: the essence of each is understood by the reader without any necessary connection to actual existence. The example of a unicorn is also applicable here. When the word ‘unicorn’ is used it signifies a concept whose definition amounts to ‘a horse with a horn extending from its forehead.’ However, despite the comprehension of the concept of the essence in question here, there was no necessary reference to its actual (or not) existence in reality in order to comprehend it. From this, Aquinas concludes that “existence is something other than the essence or quiddity.”  This is what is meant by the esse-essentia (existence-essence) distinction: that existence is not identical to essence in reference to particular existents.
This difference between existence and essence seems to be predicated on the composition of the particular existent in question, since there would only not be such a difference if there were an existent whose essence would necessarily entail existence or, to use Aquinas’ language, “something whose quiddity [or essence] is its very own existence.”  Correspondingly, any such existence would necessarily be a simple being—that is, not composed of parts—since “the essence of a simple thing…cannot be signified except as a whole, as in this case…the quiddity of a simple thing is the simply thing itself.”  Such an existence is, as Aquinas famously states, ipsum esse subsistens—that is, self-subsistent being itself.  Conversely, in a composite being the essence is really distinct from its existence since the essence does not necessarily entail existence. This was previously discussed with regard to unicorns: the essence of the word (i.e., the concept) is understood without any necessary reference to its actual existence. Therefore, these two things (namely, existence and essence) are not identical in a composite being.
From this, we can ascertain a few things with regard to the relation between existence and essence: first, in a simple being there is a real identity between existence and essence, such that the two signify the same thing—the existent itself; second, in the same there is a conceptual distinction between existence and essence, since the concept of ‘to be’ and ‘to be a sort’ are different but in this case they have the same referent; third, in a composite being there is a real distinction between existence and essence, such that they two signify different things—the intrinsic principles by which a thing is and is a certain sort; fourth, in the same there also is a conceptual distinction since the difference between the concepts remains.
There is also a connection between the esse-essentia distinction and the act-potency distinction, which shows how being, existence, and essence relate to actuality and potentiality as states of being and intrinsic principles. Esse, or the act of existing, is labeled as corresponding with actuality since it is that by which a thing is. This is what is meant by calling it an intrinsic principle: esse is an actuality whereby any given thing exists, or is in act. The state of being-in-act is co-referential upon the possession of esse. For this reason, it is the actuality of all actualities, since any act is metaphysically dependent upon this first act of esse. In this sense, esse is the very core of “being” since being, according to its proper sense, “posits something in reality.”  And something only exists in reality insofar as it has the intrinsic principle of esse, the first act of all actualities.
Essence corresponds to potentiality since it delineates what sort a thing is without per se any reference to its actual existence. This goes back to Aquinas’ argument for the esse-essentia distinction in the first place: that which the act of existence refers to necessarily posits something in reality, whereas essence does not necessarily posit something in reality. However, there is more to essence’s correlation than a further elucidation of the original distinction. In correlating essence with potency, essence is then said to limit esse by virtue of their union in the form of composition. This mirrors the act-potency relation, since act (while not metaphysically dependent upon potency) is limited by its composition with potency.  Now, in this relation of esse to essence, what is occurring is that the essence is brought into being (or posited in reality) by virtue of its union with the act of existence. It is important to note that these two intrinsic principles are contained within a particular existent: they remain distinct but united since the existent is composed of them. This is important for understanding the precise nature of essence as limiting the act of existence: the act of existence is, as it were, indefinite. This is clear from the fact that esse in the Latin is the infinitive form of the verb sum, meaning ‘to be’ as well as from Aquinas’s usage.  This is why I previously described it as the very core of “being,” since it is, above all else, that which is posited in reality. Since this act of existence is per se indefinite, it can only be limited by the essence to which it is united. The act of existence is concretized, as it were, by its composition with a given essence. This essence, then, becomes the subject of existence or, more precisely, it is the individual instantiation of the essence which exists. This is called individuation in medieval philosophy. The essence, therefore, seen in itself as in a state of potency is actualized by its union or composition with esse.
– Michael Hall
 For an overview of being qua being as the subject of metaphysics see Thomas Aquinas, Commentary on Aristotle’s Metaphysics, IV, lectio. 1. https://dhspriory.org/thomas/Metaphysics4.htm#1.
 For an overview of the act-potency distinction see Thomas Aquinas, On the Principles of Nature, c. 1. https://aquinas.cc/369/371/~67.
 Thomas Aquinas’ On Being and Essence is accessible here:https://aquinas.cc/364/366/~121.
 Aquinas, On Being and Essence, pr. https://aquinas.cc/364/366/~1.
 See Aristotle, Metaphysics, V.7. http://classics.mit.edu/Aristotle/metaphysics.5.v.html.
 For an overview of Aristotle’s ten categories see Louis F. Groarke, “Aristotle: Logic,” in Internet Encyclopedia of Philosophy, eds. James Fieser and Bradley Dowden, https://www.iep.utm.edu/aris-log/#H2; Joe Sachs, “Aristotle: Metaphysics,” in Internet Encyclopedia of Philosophy, eds. Fieser and Dowden, https://www.iep.utm.edu/aris-met/#H5. For an overview of Aristotle’s Categoriess ee Paul Studtmann, “Aristotle’s Categories,” in The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), ed. Edward N. Zalta, https://plato.stanford.edu/archives/fall2018/entries/aristotle-categories/.
 Aquinas, On Being and Essence, c. 1. https://aquinas.cc/364/366/~5.
 Ibid., c. 4. https://aquinas.cc/364/366/~74.
 Ibid., c. 4. https://aquinas.cc/364/366/~73.
 Ibid., c. 4. https://aquinas.cc/364/366/~74.
 Ibid., c. 1. https://aquinas.cc/364/366/~6.
 See my previous post on the act-potency distinction. https://thereformedphilosopher.com/2018/12/25/thomistic-metaphysics-ii-the-act-potency-distinction/.
 See “Introduction,” in Thomas Aquinas, On Being and Essence, 2nd Ed., trans. Armand Maurer (Canada: Pontifical Institute of Medieval Studies, 1968), 7-27.