Let’s take abstraction as given. We sense particulars and form universals from them by way of drawing out various common features. The abstraction, however, has to leave aside the concretion and therefore existence of the particulars.
But this leaves existence without a home. The universal abstracts from it, and yet existence is clearly common to many things and so can’t be located in the particular. The particular can be given to the mind as some sort of correlative to universality, but existence is distinguished from both terms of the relation.
Neo-Scholasticism described existence as known by an “imperfect abstraction”, though the description might hide more than it reveals. The term suggests that being is given by a defective act of abstraction or that, if one could only abstract more perfectly, he would have no idea of being at all. To say the least, this paints the wrong picture of what’s going on.
The Laval school sometimes complained that being as such is the most vacuous and empty of concepts, and that to merely appreciate things as beings is to have a radically imperfect knowledge that needs to be perfected by concretion. If this were all there were to it, it would be one way to understand existence as given by “imperfect abstraction”. But the argument confuses the order of demonstration with the order of universality. Understanding something as a being can be taken in two ways, either as a vague, imperfect awareness of its existence, or as a subject of metaphysics. In the first sense, being is an imperfect concept, in the second its degree of perfection depends on the level of sophistication with which one has explicated it in its proper science. To merely understand that an elephant exists tells us almost nothing about it, but the degree to which our understanding of existence is perfect (even the existence of elephants) is a function of how far we have progressed in metaphysics.
True, being is given somehow by abstraction in the sense that it involves brain activity, but it cannot be just taken as the most universal concept without leaving something crucial out. One way forward would be to see being as latent or habitual in human knowledge, and only actualized by sense activity and abstraction, though with a different root. I Sentences 3.4.5. might be helpful, or the other places where St. Thomas deals with the soul’s knowledge of itself.