We know the thing in itself by a direct and intuitive perception of its nature.
We know the thing in itself by participation in its nature.
A man knows the spirit of all men because all men share the same nature.
Thus, it is possible to know another man even as it is possible to know ones own self.
To nature we add what the Greeks call the hypostasis, which is a bit like the word “instantiation”. This is related to the idea of the Person, though not exactly analogous to it.
To put one way is to say that a nature (of any kind) is never known apart from a hypostasis. Thus, there is no “human nature per se” but each human person partakes in human nature along with all other human persons, and none of their persons accounts for the totality of human nature. It is a participant in a unity.
This unity of nature, and multiplicity of persons, is the basis for Personal Reality.
And so we know reality, the thing in itself, through participation in its nature, but also through the knowledge of persons.
Because basis of reality is not simply a “what” it is a “who”.
So there is unity and multiplicity, and the one cannot be found without the other. They arise/exist simultaneously.
This unity of nature, and multiplicity of persons, is directly related to the Triunity of the Godhead, which is One Divine Nature expressed as three unique Persons.
If God were only one person, there would be no person at all, only the nature, because persons can only arise in the context of other persons. And since nature has no expression except through an hypostasis, there would, then, be nothing at all.