So I've been having an argument with a buddy of mine for years now over this topic.
Is nothing, to the literal sense, a thing?
Or take this anon as an example. If he believes in an imaginary nonexistent being, how certain can you be that said being doesn't exist? It at the very least exists as a concept in his head
You're pretty much repeating Descartes. You can negate anything except the act of negation. Negating negation only affirms its existence. So if the act of negation is the only thing that can't be negated then negation and the conscious thought it stems from is the proof of your existence. Thus, cogito ergo sum - I can think reasonably therefore I am.