On the conceptual undefinability of being
“The subject matter of metaphysics can be put in a single word, then, and that word is ‘being’. Because this is so, it would seem to be most appropriate if we were to provide a precise definition of the word. Unfortunately, any attempt to do so would meet with frustration. ‘Being’ cannot be defined, not in any strict sense, at any rate. To see why this is so we need to call to mind what is required in order to effect a strict, or logical, definition. There are two steps to the process. The first step is to place the term to be defined, the definitum, in what is called a proximate genus. Secondly, we cite the specific difference, certain properties of the definitum which set it apart from other members of the genus. So, for example, in defining ‘man’ we place the definitum in the genus ‘animal’, then we cite the difference ‘rational’, which, in rather dramatic fashion, separates this particular kind of animal from all other kinds. And our finished definition is: ‘Man is a rational animal’.
“What would be the result of our applying this procedure for definition to the term ‘being’? Our first step would be to place the definitum into a proximate genus. A genus is simply a class of things; a proximate genus is one whose members are most like, in terms of their essential features, the definitum we are dealing with. But what would serve as a genus for ‘being’? If we were to cast about looking for a class into which we could put our definitum, we would not be able to find one. Because everything is a being, there is nothing outside being into which it could be put. So, we cannot even make the first step in the defining process, if our definitum is ‘being’. And even supposing that we could have succeeded there, we would be blocked from taking the second step. The reason for this has to do with the fact that in order to identify the specific difference, we have to go outside the genus in which we have established our definitum. For example, in defining ‘man’, the specific difference ‘rational’ was not to be found within the genus ‘animal’. If that were the case, then every animal, by dint of the fact of being an animal, would be rational, because rationality would be an essential feature of an animal. Applying these considerations to our attempt to define ‘being’, we see that our definitum cannot have any specific difference applied to it, for outside being there is literally nothing.
“But logical or scientific definition is not the only kind of definition. It admits of a looser form, called descriptive definition, and St. Thomas provides us with a remarkably succinct and revealing descriptive definition of our subject matter. Being, the Universal Doctor writes, ‘is that whose act is to be’ (id cujus actus esse). In just four words he directs our attention to the two key aspects of being: the subject of existence (id), and the act of existence itself (actus esse). We might expand that definition by saying that it is the very meaning of a being, its core identifying character, to be, actually to exist.”
— D. Q. McInerny
Metaphysics (Priestly Fraternity of St. Peter, 2004), pp. 39–40.
