Whenever we have two concepts, A and B, we can ask to what extent the things they pick out in reality are distinct. If they pick out distinct realities, then we say that there is a real distinction between them. If they pick out the same reality, however, then we say that there is a real identity between them. Even if two concepts are really identical with one another we can still meaningfully talk about a distinction between them, and Thomists say this can happen in two ways.

A conceptual (or merely logical) distinction is when the two concepts pick out the same reality in every way, and the only distinction to be had is in the way we’re considering that reality. For instance, Superman and Clark Kent are conceptually distinct from one another. There’s nothing true of Superman that is not also true of Clark Kent, and vice versa. Another example is a particular incline that is understood as either a downhill or an uphill. These are the same thing considered from different perspectives.

A virtual distinction arises when the two concepts pick out the same reality, but where this reality is understood with respect to two other really distinct things. In other words, we say that A is virtually distinct from B when (1) both A and B pick out some reality Z, (2) A is Z understood with respect to some C and B is Z understood with respect to some D, and (3) C and D are really distinct from one another.

We saw some examples of virtual distinctions when discussing potential wholes recently, and we’ll repeat two of them here. First, faith is thinking with assent. Of itself, faith is a single action, but it has an intellective aspect (thinking) and a volitional aspect (assenting) each of which involves the use of a different power (the intellect and will respectively). These two aspects of faith, then, are virtually distinct from one another, because they are the same act understood with respect to distinct powers. Second, a water molecule arises from a single bond configuring two hydrogen atoms with one oxygen atom. Now, we can consider the configuration of one of the hydrogen atoms, and we can consider the configuration of the oxygen atom. These two concepts pick out the same underlying reality — the configuration making up the whole water molecule — but do so with respect to distinct elements of the water molecule. As such, they are virtually distinct from one another.

These, then, are the two non-real distinctions, and in each case we could say when such a distinction occurs. Can we do the same thing for real distinctions? One common proposal is that two concepts are really distinct when the realities they pick out are separable, that is when one can exist without the other. Now, certainly separability is a sufficient condition for a real distinction, but is it a necessary condition? For Thomists the answer is no, since we think that a real distinction can occur between inseparable things. In cases where two things are inseparable, then, what is the condition that accounts for their real distinction?

I want to suggest that what we said about virtual distinctions can help us answer this. Looking at the three sub-conditions I listed for virtual distinctions, the second is critical and what links the other two. It is because being understood with respect to C does not exclude being understood with respect to D that there can be one reality picked out by the two concepts. If one of these relations did exclude the other, then the two concepts must pick out distinct realities, and therefore be really distinct. We’ll call this the exclusion condition to distinguish it from the separability condition.

Now, if the exclusion condition is to be of value to us it cannot apply in all and only those cases the separability condition applies. There are clearly cases where the two conditions coincide. To give a simple example, let A pick out me thinking something, and let B pick out me thinking the opposite. Assuming I’m not beset with doublethink, these two realities exclude one another. And they are certainly separable from one another. To find a case of exclusion without separability we need to look a bit harder. Perhaps the most famous (or infamous) example is the distinction between essence and existence in created beings. Aquinas argues that this is a real distinction, despite the two being inseparable from one another. His argument is fairly involved, so here we’ll just sketch enough for illustrative purposes.

Sherlock Holmes and I have a number of important things in common. We are both composites of form and matter, for instance, and we have similar sets of natural powers, even if he has some of these to a greater degree than I. The most salient point is that we share a common essence, on account of which we are both called human and by which we are distinguished from other kinds of substances. As far as I’m aware, however, I exist and he doesn’t. What this means is that our common essence itself cannot differentiate between an existing human and a non-existing human. Put another way, our essence of itself is indeterminate between existence and non-existence. I exist, then, because my essence has something else added to it which determines it to existence rather than non-existence. This something else is called esse in Latin, and is variously translated into English as “being” or “existence.”

All of this might sound like a convoluted way of saying what amounts to the tautology that I exist because I have existence. But such a complaint rides on an ambiguity. When I speak of a common essence shared by myself and Sherlock I do not have in mind some abstract universal that lies outside of each of us, but rather the particular feature found in each of us in virtue of which each of us fall under that universal in the first place. To illustrate the difference between these two consider the simple example of two groups of wood, each organised into a square shape. In this picture there is (1) the universal squareness which is instantiated twice, (2) the particular square organisation which is in the first group, and (3) the particular square organisation which is in the second group. It is in virtue of each of the groups having the organisation in itself that it can fall under the universal in the first place. So too with the common essence shared by Sherlock and myself.

Just as my essence is in me, so too its determination to existence is in me. It is because my essence is determined by esse and Sherlock’s is not that I exist and he doesn’t. So, then, our earlier conclusion really amounts to the non-tautologous claim that a certain fact about me (that I exist) is true in virtue of some feature in me (my esse).

Now, the argument I ran with myself and Sherlock can be applied to any being, so that all beings exist in virtue of esse within them. Esse, therefore, accounts for the similarity between all existing beings insofar as they exist, which is to say it unifies all existing beings qua existing. Essence, on the other hand, diversifies and differentiates these beings from one another, by qualifying their existence in different ways. For instance, two beings A and B are similar to each other in that they both have esse and thereby exist, but differ from one another in that A’s essence makes him an existing human whereas B’s essence makes him an existing angel. The essences of material beings additionally requires that their existence be qualified to a place and time, which allows multiple beings of the same species to exist.

Since esse unifies and essence diversifies, it follows that these two concepts exclude one another. And since a being can’t exist without its essence and esse these two are also inseparable from one another. So we have an example of a real distinction on the basis of exclusion without separability.

Before we close, we must introduce an important nuance. Strictly speaking, all that is needed for A to be a distinctly existing being from B is for A’s essence to qualify its existence in a way that B’s does not. Notice, however, that this leaves open two options regarding B’s essence: either it qualifies B’s existence in a way A’s essence does not, or it doesn’t qualify B’s existence at all. In the latter case, B’s essence would do nothing to exclude it from being really identical with B’s esse. Nevertheless, it is clear from the foregoing that at most one being can have unqualified existence, and so in all other beings there will be the real distinction between essence and esse we’ve been talking about.