Thomism teaches that we can know that God is but not what he is. The same limitation characterizes anything not given in sense intuition, and so the whole of metaphysics as such, including substantial things like angels and soul and all else that can characterize them per se like causes, being, goodness, person, dignity, free choice, etc.
But how can I claim that I don’t know what God is when I say so many positive things about him like cause, person, good, etc? Don’t I have to know what I am talking about before I show you whether or not there is such a thing?
By “knowing what” Aristotle and STA mean defining or at least making the first move in defining, and the first move in defining is specifying a genus. Inability to know what something is therefore = inability to locate it in a genus.
So if you wanted to claim this:
Everyone who knows that something is knows what it is
You’d have to claim this:
We can locate everything we know in some genus.
But the possibility of category errors make it clear that there are irreducible genera, and so we can recognize the likeness among things (genera) without reducing them to some genus. If all knowledge placed things in a genus we could not know distinct, irreducible genera at all. From this perspective, all knowledge of the homogenous is contextualized in knowledge of the non-homogenous.
If all likeness were homogenous then we would get a version of the third man argument, i.e. if A and B could only be alike in virtue of a genus C, then even to speak of a likeness between this genus and that one would require a third genus which, qua genus, would be itself like the other two and so require a fourth and a fifth, a sixth and a seventh, and so on without ever explaining anything.
If not every likeness is homogenous, then likeness is not in a genus. Nevertheless, one can use the term genus to as describing any likeness, as when Aristotle says that the dispute over “whether pleasure is good” is a dispute about its genus. This use of the term is so loose as to be metaphorical.
We might visualize diverse genera as Venn circles in distinct spaces, but this is a metaphor for something far more interesting. The “space” of these circles is our knowledge that something is without knowing what it is, IOW, the space of these Venn circles is our knowledge of things like God, free choice, angels, persons and (of course) being. In the order of discovery, the knowledge of this common space comes only after our knowledge of things in genera, but as a structural or non-conscious possibility things are very different and our knowledge of things like God is the context in which sensible things can be known.