In debates (or even just discussions) people sometimes ask how we know the physical world is really there. A variation asks how we know that what we perceive as the real world is the same as what other people perceive. (One example of this is the inverted spectrum.)
The most accurate answer is: We don’t. Not for sure, anyway. There is at least one assumption built in, but it’s one we have to make to escape our own minds. According to ancient philosophical tradition, the only fact we know for sure is that we ourselves exist. (Although I think there’s an argument to be made about a priori knowledge.)
But, as with the excluded middle, accepting reality as an axiom seems almost necessary if we’re to move forward in any useful way.