On the one hand he can sometimes appear to reduce the predication of esse as 'actuality' to near vacuousness - specifically in denying that there is any 'concept' of esse, for in the sense in which there is a 'concept' of man, or of cheese, there can be no 'concept' of existence. Concepts are expressed in descriptions which tell you what something is, whereas to say that something 'exists' is not to say anything further about what it is. And this is, of course, right. For the difference between chalk and cheese, or between any two kinds of thing, is not at all the same kind of difference as that between either of them existing and their not existing. The sort of difference involved in 'This isn't cheese (it's chalk)' is quite different in logical kind from that involved in 'There was some cheese, but there isn't any cheese left.' The first is a difference of kind, and is what we refer to when we say that there is a difference of 'concept'. The second is not some difference in kind and cannot fall under any concept at all.
It is therefore no use looking for some same attribute additional to what all things are by way of tracking down what they have in common as existing, as you do when, knowing that some things are animals, you look in addition for whatever will tell you whether they are humans or brutes -that is, by looking for the presence or absence of signs of rationality. Nor do you see a thing's esse by observing whether there is the sort of difference between its existence and its non-existence which there might be between two things which differ in colour, or weight, or size. Knowing what or how a thing is, you do not get at a thing's esse by staring at it a bit harder so as to glimpse something else about it that it possesses in common with everything else actual, and which you might have so far missed. To put it in Thomas's terms, a thing's esse is neither its substantial nor its accidental form, nor is a thing modified by existence's being predicated of it in any of the ways in which a thing is modified by form, for esse is the actuality, he says, of all things 'and even of forms themselves'.12 And in this, too, Thomas must be right. For again, were a thing's esse to make any formal difference to it, then it could not cease to exist, for what first existed and then no longer does would not be the same thing. Hence, there cannot be a difference in form between an existing and a non-existing x.13 Esse, therefore - not being the object of any concept - cannot be predicated univocally, for were it predicable univocally that could be in terms only of some same formal characteristic predicated of all things said to exist. That, essentially, is the mistake of Duns Scotus.
Moreover, Thomas presses the point so hard, and with such a degree of firmness, as would appear to place at risk another doctrine central to his account of esse, namely that in any created thing there is a real distinction between its esse and its essence. Even if esse, he says, is not a formal attribute of what exists, esse is predicable only as a function of some form.14 A sheep's existence is an ovine existence, a cow's a bovine, a man's a human existence, or, as Thomas says more generally, 'for a living thing to exist is for it to be alive'.15 And it follows from this that, when you speak of the existence of anything, you speak of much more than the existence of just it: when you say that x exists you are saying that there exist (are 'actual') all those conditions which must obtain - the sort of'world' - such that that kind of thing can exist in it. A sheep cannot exist without an ovine world - requiring (at any rate until recently) there to have been at least two other pre-existing adult sheep, one male and one female, and requiring a whole range of other conditions, atmospheric, chemical, biological, environmental, and so forth, such as permit the possibility of the kind of thing a sheep is to exist at all. To give such an account is to engage in the forms of scientific knowledge which explain what it is like that there should be sheep - or, as Thomas puts it, it is to know the answer to the question quid est?, the answer to which yields knowledge of its 'essence'. But you do not get at a thing's actuality by any other means than by knowing in such ways the essence which it
13 It was such considerations which led Kant to say that 'existence is not a predicate'. But, as we have seen, there is no need to deny that existence, as actuality, is predicated of what exists to avoid the absurd conclusion in question.
14 'Esse... per se convenit formae quae est actus.' ST 1a q75 a6 corp.
15 'Vivere viventibus est esse'. ST 1a q18 a2 sed contra. Thomas refers to Aristotle, De Anima II.4, 415 b13.
actualises. To put it in other terms, what you predicate when you predicate esse of a thing is the value of the variable 'x' in 'x exists'. It is for this reason - that existence is predicable only as a function of some form - that there can be no univocal predication of esse as between different kinds of thing.
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