Stephen asked:

What is a thing?

A dictionary defines a "thing" as "whatever is or may be perceived, known, or thought about".

However, is it philosophically possible for there to exist a "thing" that is impossible to be thought
about (given, for instance, the limitations of the human mind)?

If so, what kind of "thing" may this be? Could it be possible to predict, say, the 'nature' of such
"things"?

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First of all, I can think about the limitations of the human mind, in particular my mind. And so can you.

And, in fact, people are not ordinarily called "things" in polite society, so the dictionary seems to be
wrong

But your question is a good one.

The word "thing" is a kind of place-holder for whatever you want to put into it. It is a deliberately
vague term used to avoid specificity. For instance someone who is leaving might say, "I have to get
my things together" to avoid saying, "I have to get my coat and my hat and my sweater...." which
would be tedious and unnecessary. It is a useful term. And its use teaches us that there are times
when we have good reason to avoid being specific. "Look at all the nice things there are to eat!" again
is said so as to avoid having to list those things.

Ken Stern