The idea is that one and the same item — humanity in our example — can exist in two ways. It can exist in particular concrete things outside the mind, and it can exist in an abstract and universal form in minds. But in and of itself it is neutral as between these two modes of existence. Taken by itself, therefore, it does not exist, and is neither particular nor universal. In itself, it is neither many in the way human beings are many, nor is it one, in the way in which the universal humanity in the mind is one.
So this Thomist essence is an item that is some definite item, though in itself it does not exist, is neither one nor many, and is neither universal nor particular. I hope I will be forgiven for finding this unintelligible.
The best way I have found to think about essence in the Thomistic sense is as a way of being. I see the street sign in front of me that says Porter St.; I notice that it is in the form of a rectangle, and therefore has four right angles. To that extent, at least, the street sign has being in the way of four. It is also a physical being located at a certain place in time and space, and is subject to material division, and so it has being in the way of body. I could keep going along these lines, describing the many different ways in which the street sign manifests being. But, no matter how far I go along these lines, I am not bringing anything into existence in describing the variety of ways of being. In other words, a way of being is not itself a being, at least in the sense that the street sign is a being. A way of being is just that, a way of being. It is the difference between the plans for Fenway Park and Fenway Park itself. The plans for Fenway Park describe one way of being a baseball stadium; Fenway Park itself is a being that is being in the way of its plans. Similarly, four is a way of being or a plan of being; the Porter St. sign is an actual being that is being in the way of four.
We describe the Porter St. sign according to a variety of ways of being, but what of that sign in itself? In itself, it is not an amalgamation of ways of being; it is what it is simply. The analysis of being in terms of ways of being is a peculiarity of the way of knowing of a rational animal (our way), a way St. Thomas calls "composing and dividing." Our nature is not such that we can know being simply and directly; we can't immediately know the way of being the Porter St. sign. Our initial impression of being is confused and opaque ("Something is there, but I don't know what it is...") and, over time, as we analyze being in terms of its ways, it unfolds its nature to us. But it would be a mistake to confuse the multiplicity in our way of understanding being with a multiplicity in being itself.
That can happen if we confuse the two meanings of the word is. As St. Thomas discusses in his On Being and Essence, we use is (being) in two ways: In one way, as a fundamental existential predicate, and in another way, as indicating truth through a relationship of ideas. In my terms, the first way of using is is when it is used to say that something is actually fulfilling a particular way of being, and the second way is when it is used to express relationships among ways of being. For example, if I say the Porter St. sign is four in the second way, what I mean is that the way of being the Porter St. sign includes the way of being four. If I mean it in the first sense, I mean the actual Porter St. sign is actually fulfilling the way of being four. Similarly, If I say Hamlet is a man in the second way, I mean that Hamlet's way of being, if he is, is that of a man. I can say the same thing in the same way, with the same meaning with respect to Socrates: Socrates is a man. But in the first way, the statements are not equivalent, because Socrates actually fulfills the way of being man in a manner that Hamlet does not, since Socrates is an actual man and Hamlet only a fictional character.
Much of philosophical history can be traced to not getting this right, as Etienne Gilson demonstrates in his Unity of Philosophical Experience and Being and Some Philosophers. It is tempting, on seeing that Socrates is a man and Plato is a man share the idea man, that the beings of both Plato and Socrates "participate" in the idea of man, which is itself some third thing above and beyond the beings of either Socrates or Plato. The temptation results from the failure to take account of the multiple meanings of the word is.
Returning to the Maverick Philosopher, he wonders about the nature of the existence of the "item" called "humanity." But humanity is not an item, as though it has substantial being in its own right. It is only a way of being, and has existence either in its fulfillment in actual men, or in abstraction as a plan for being. To wonder about some third way it might exist is to misunderstand the nature of essence. Similarly, the plans for Fenway Park exist in the actual Fenway Park by way of fulfillment, or in blueprints in the builder's office. There doesn't need to be any third way beyond these ways to make sense of things.
Essences are universal in the sense that blueprints universally apply to whatever actual things are built according to their plans. But blueprints are only useful as a means to an end, and essences in the universal sense in which they exist in the mind are only useful as a way for the human mind to know being. An intellect, like that of an angel, that knows being simply and directly has no need of universals.