Sure you can say that everything I see is technically a representation in my head. But there is very much a shared reality, a domain of objects and state of affairs for which we both agree about the presence or absence of the same representations. There's a difference between a value and our measurement of a value. If every multimeter we touch to a battery reads 5.9v, its possible the battery is actually 6v and every single multimeter was coincidentally wrong. However from the inside the situation is indistinguishable from a 5.9v battery. We may as well accept the reality of our perception and its representations as the real thing per se, because even if it is delusion we are still stuck playing by its rules as if it were real.

I am "really" seeing an eye in the mirror in the sense that I'm seeing the same thing other people would call an eye, and thus it exists in the shared domain. To have a shared domain of objects and facts is to have a common ground. There is no "object itself" for us to reach out and grasp outside of our mutual perception of objects. We might all be in a computer simulation. Doesn't matter. The conversational/perceptual reality is reality.