The only thing that anybody knows is their own mind. My opinions are what I know them to be; this is not subject to debate.
Everything else is debatable. People don't know that we breathe air. We could be brains in vats, or computer simulations, or some such thing. That's far-fetched, I know; I don't think we're brains in vats, but the point is: we don't know that we're breathing air; we only know that we think we're breathing air.
Defining knowledge in this way is less problematic compared to every other way. Other ways involve degrees of certainty, and/or correctness. The more certain one is about the nature of reality about which one has supposed knowledge, the more likely it is to be considered actual, but one can feel one hundred percent certain, and still be mistaken, or one can feel completely uncertain, take a wild guess, and prove correct.