What can all 7 billion people agree on?

Agree on? Or rather acknowledge?

I could, for example, propose that everyone would know that they "are" but, there are certain schools of thought that deny even that.

It would be something ultimately subjective, but a singular statement nonetheless, if every human out there is to agree with it.
They could acknowledge that their frame of reference is subjective, but they may not explicitly state it, or agree when it's proposed.

Or, these 7 billion can all mutually confirm that my post didn't add anything to this discussion.