According to the Thomistic view in Ontology, essence and 'the act of being' (I'm translating from a romance language) are different, in which 'to be' is the act received by the essence, which is the potency that limits the act of being.
So, my question is: if the act of being comes from God, who is Pure act of being, then where does the essence/potency come from?
Any help would be appreciated. ;)
I have not studied St. Thomas but the theory is also discussed in muslim philosophy, that's why I readily recognized the theme. My answer also comes from my knowledge of muslim philosophy but I think due to common origins between Medieval Christian and Muslim philosophy the theories must be identical.
Now to answer your question, the essence/potency doesn't have to come from anywhere, because they are nothing at all in the first place! That is, essence as limit of being is itself non-being or non-existence. Because only non-existence limits existence, while the act of being by God only creates being/existence.
I think infatuated mostly explains it well, but I wanted to pitch in a couple points:
One alternate way of thinking about/translating the same statement ipsum esse subsistens is God as "unconditioned being." When one asks "where does [God's] essence/potency come from?" one puts conditions on it (as in "we are talking about the essence that comes from X..."). Another translation is "the infinitive 'to be' existing by itself," and asking where it comes from doesn't make sense because that implies a past, which implies a conjugation of "to be," which violates the definition.
According to Thomas, for the rest of us, our essence comes from God/ipsum esse subsistens. That is, we exist, but in a conditioned state.
Probably one of the greatest theologians of our time, Carl F.H. Henry, citing John Calvin, writes:
The knowledge of God is nonetheless for the sake of the good life. "We are invited to a knowledge of God not such as, content with empty speculation, merely floats in the brain, but such as will be solid and fruitful, if rightly received and rooted in out hearts" (Institutes I, 5, 9). The pious mind covets knowledge of God for the sake of reverent worship and obedience. "The right way, and the best method of seeking God" is "not with presumptuous curiosity to attempt an examination of his essence, which is rather to be adored than too curiously investigated; but to contemplate him in his works, in which he approaches and familiarizes, and, in some measure, communicates himself to us" (Ibid, I, 5, 9). "Cold and frivolous . . . are the speculations of those who employ themselves in disquisitions on the essence of God, when it would be more interesting to us to become acquainted with his character, and to know, what is agreeable to his nature" (Ibid. I, 2, 2).
In other words, "Don't look for God to take a shower." Or, better, "Don't peak at God lounging poolside."
(I would highly recommend Henry's work in Christian theology, God, Revelation, and Authority [in five volumes: 1976-1987]. He address the problem of Thomastic theology, at least partly, in his Chapter 20: "The Theological Transcendent A Priori," Volume 1.)
Nevertheless, Plato's concept of 'Form' or 'eternal substance' (eidos) is related significantly to 'essence' (ousia); also, hypostasis is substantial, concrete being between the merely actual but contingent and the realities that are merely mental. The whole picture is a 'tricotomy':
mental reality (appearance)
hypostasis: concrete being (substance)/becoming (change): 'reality'
actuality by contingency (hypokeimenon): the basic primary matter which has not yet been formed: a circling back to the original Form (Ideal). (In successful philosophy and philosophic-theologic world-life views: that which is placed in the lower story does not consume the upper (as in modernism and postmodernism); rather, it merges with it. Watch for this.)
Historic or personal movement through time begins 'top-to-down'. Reality and appearance are sometimes opposed. Time (to the Platonists and Neo-Platonists) was thought of as noema (thought or concept) and metron (measure), but not as hypostasis (the reality we experience; with this in mind, you might detect how C. Godel [Einstein's best friend] was merely a Neo-Platonist).
Hypostasis is further identified with the world spirit or world soul. It is 'essence' that gives stability to the concrete, individual thing. This is the world of substantia (substance: electrons, protons, neutrons and stuff; This is my, your world; the objective world).
Plato's 'Form' basically meant "to place under." Under what? Under heaven, God's realm. Our world is both under heaven literally and by authority (the Creator's conduct). Our world is like a world under water... a bottom world upon which one can or cannot get a foothold. It's like 'night-floundering' in Georgia swamp land, a lantern and a 'gig' in-hand. You look, feel for solid ground under and through the deep dark, dank forested swamp waters. Sometimes nothing is there, so you float. Or, you tread water (with all that stuff in both hands, better tread with your feet)! Sometimes you might feel with your bare feet the muddy bottom ooze. Yea, "That's 'bottom'." Tillich's "God, the ground of all being."
(I would highly recommend The New International Dictionary of New Testament Theology [1967; 1975], Volume 1: s.v., "Form," p. 703-714.)
To complement user infatuated's answer, and since many may not read my comment.
Roughly translated, it goes as follows:
Essence only exists by being, since before having 'being', it is nothing, except only in God's 'Understanding'[??], where it is no longer a creation, but the Creator's essence. God, when creating, produces beings from nothing, as an act limited by its essence [the act's corresponding essence]; not from two different things that were separately made, and then joined, but only one thing, a compound of both potency(essence) and act(being).
[??] I'm not exactly sure how to translate the expression written in the original.
