On Being and Essence

Overview

About eighty pages, written by Thomas Aquinas as a young man, probably between 1252 and 1256, while he was lecturing at Paris as a bachelor rather than yet holding the rank of master, for the use of his fellow Dominican friars. It is one of the earliest works in his corpus and, despite its brevity, is generally regarded as the single most compact and complete introduction to the metaphysical framework that underlies everything he wrote afterward.

The title states its subject exactly: the treatise examines what the terms being, ens, and essence, essentia, mean, and how they apply across the whole range of existing things, from the most basic material substances up through immaterial created beings to God. Aquinas structures the inquiry by working through essence’s presence in progressively different kinds of things, using each case to sharpen a single underlying distinction he takes to be the key to the whole subject.

The work draws heavily and explicitly on Avicenna, the great Persian philosopher whose adaptation of Aristotelian metaphysics, transmitted into Latin through translation, gave Aquinas both the technical vocabulary and, on his central claim, the philosophical content he develops here. Aquinas is working within a tradition already partly formed, but the precision and systematic completeness with which he lays out the distinction between essence and existence in this short text exceeds anything available to him in his sources.

The argument

What essence means, and where the inquiry has to start. Aquinas opens by noting that essence, sometimes called quiddity, from the Latin quid, what, is what a thing’s definition expresses, the answer to the question of what kind of thing something is. Because a definition must include the genus and specific difference that together state a thing’s nature, essence functions as the ground of a thing’s intelligibility, what makes it the determinate kind of thing it is rather than something else.

He notes immediately that the word being is used in two principal senses relevant here: being as divided into the ten Aristotelian categories, substance and the various accidents, and being as signifying the truth of propositions, whether a statement affirming something is actually the case. His inquiry into essence concerns primarily the first sense, being as found in substances and their real natures.

Essence in composite material substances. Aquinas begins with the most familiar case, ordinary physical things composed of form and matter together, like a human being or a stone. Here essence includes both form and matter, not matter considered vaguely or in general, but this particular flesh and these particular bones as the kind of matter proper to a human nature specifically. This is why, Aquinas argues, a human being cannot be adequately defined the way a mathematical figure can, by form alone; the definition of a natural substance like human being must include reference to matter as part of what the essence actually is, since a human soul informing no matter at all would not be what we mean by a human being.

He works through several subtle questions here: how essence relates to genus and species, how individual members of the same species share one essence while remaining numerically distinct individuals through their particular matter, and how universal terms like animal or human relate to the essences of the particular things they are predicated of.

Essence in separate substances, and the central argument. Aquinas then turns to beings that, on his account, exist without any matter at all, the separate intelligences that medieval philosophy, following a tradition combining Aristotelian cosmology with Christian angelology, identified with angels. Since these substances have no matter, their essence cannot include matter the way a human essence does; an angel’s essence is simply its form, without further composition of form and matter.

This raises the treatise’s central and most consequential question. If an angel’s essence is simply its form, with no matter to compose with that form, is an angel therefore entirely simple, without any composition at all, identical in this respect to God, who is also understood as purely immaterial? Aquinas’s answer is no, and the argument he gives here is the philosophical heart of the entire work.

He argues that essence and existence, esse, are genuinely distinct even in an immaterial being like an angel, and his central proof turns on what can be understood. The essence or quiddity of any created thing, he argues, can be fully and completely understood by the intellect without the intellect thereby knowing, or even being certain, whether that thing actually exists. You can grasp completely what a phoenix is, or what a particular mathematical figure is, without knowing whether any phoenix or any instance of that figure actually exists anywhere in reality. If essence and existence were simply identical in a thing, understanding the essence completely would automatically include knowing that the thing exists, since you cannot understand something completely while remaining ignorant of an aspect identical to it. Since this plainly is not the case for any created thing, essence and existence must be really distinct in every creature, whether material or immaterial.

It follows, Aquinas argues, that even an angel, despite having no matter to compose with its form, remains composite in this more fundamental sense: composed of its essence, which is its form, and a distinct act of existing that essence does not itself include or guarantee, and which the angel must therefore receive from an outside cause.

Essence in God. Having shown that essence and existence are distinct in every creature, material or immaterial, Aquinas argues that this distinction cannot regress infinitely; there must be some being whose existence is not received from anything outside itself but which simply is its own act of existing, with no gap at all between what it is and that it is. This being, whose essence just is to exist, and who therefore exists necessarily rather than receiving existence contingently from a prior cause, is what Aquinas identifies as God. In God alone, essence and existence are identical, which is why God alone is absolutely simple, without composition of any kind, and why God alone exists by his own nature rather than by participation in existence granted from elsewhere.

Essence in accidents. A final, shorter section addresses how essence applies to accidents, the further qualities, quantities and relations that inhere in a substance rather than existing independently, extending the same basic framework, with appropriate modifications, to this further category of being.

How to read it

Read chapter one for the basic account of essence and chapter four for the central argument about the real distinction between essence and existence; together these carry most of the treatise’s philosophical weight, though the intervening chapters on essence in composite substances repay careful reading too.

Work the central argument through with your own example before moving on. Take some definite thing whose essence you can state clearly, a unicorn, a particular geometric figure, anything you can define without difficulty, and ask yourself honestly whether grasping that definition tells you whether the thing actually exists. If it does not, and it should not, you have reconstructed for yourself the core move Aquinas makes to separate essence from existence.

Notice how carefully Aquinas distinguishes the case of material substances, where essence includes both form and matter, from the case of angels, where essence is form alone, before arguing that even angels remain composite in the further sense of essence and existence being distinct. Collapsing these two different senses of composition, form-matter composition and essence-existence composition, is the most common way readers get confused, so keep them separate.

Know that this treatise’s central claim, the real distinction between essence and existence in creatures, became one of the most contested theses in the later history of scholastic philosophy. Some later Thomists treated it as the single most important and defensible insight in Aquinas’s entire metaphysics; other major scholastic thinkers, including figures working in different philosophical traditions within medieval Christendom, rejected the real distinction as unnecessary or incoherent, arguing that a merely conceptual or formal distinction between essence and existence does all the work Aquinas thinks he needs a real distinction to do. Reading the argument with this later controversy in mind sharpens what exactly is at stake in accepting or rejecting it.

Read this treatise before or alongside the relevant early questions of the Summa Theologiae, particularly the treatment of the divine essence and existence in the first part. The framework worked out here in compressed, purely philosophical form underlies Aquinas’s mature theological arguments for God’s existence and simplicity throughout his later career.

Questions it raises

• Is Aquinas’s central argument, that essence can be understood without knowing whether the thing exists, a genuine proof that essence and existence are really distinct, or does it only show they are conceptually distinguishable?
• What is actually added to an essence when existence is said to be received into it? Is existence itself a kind of thing, or is calling it an act misleading?
• Does the real distinction between essence and existence in creatures do necessary philosophical work, or could a merely logical or conceptual distinction explain the same phenomena more economically?
• If God’s essence simply is to exist, does that make existence itself, rather than any further attribute, the most fundamental thing that can be said about God?
• How does Aquinas’s claim that angels are composed of essence and existence, despite having no matter, change how we should think about what composition and simplicity mean?
• Can a philosophical argument, proceeding purely from the analysis of essence and understanding, actually establish that a necessarily existing being exists, or does it only establish what such a being would have to be like if it existed?