Wsieber: It would mean a thought can also be a thing.

Inselpeter: Not only can be, but is. Perhaps it would be helpful to ask, 'what is not a thing?

Perhaps, Mr. inselpeter you will allow me to disagree with your original answer to wsieber's question. Your qualifying removal of the period in order to use the article "a" as a designator changed the rules so this contra interpretation is valid as well. eg,
The waitress winks and smiles a knowing smile that indicates she remembers all-too-well the drunken promises that you made late last night and says to you..."Coffee?"

Notice she did not say " a coffee". The "a" was implied and understood, just as the "Do you want...".

Now to address your question...

What is not a thing?

That's easy. "Nothing" is not a thing. But the human mind cannot comprehend the concept of "nothingness" without "somethingness" to provide contrast, therefore everything is a thing including the absolute absence of everything.
Follow...?