Yes, I wholly agree with this and would argue it is the case with everything we humans argue over, we agree to define certain things the same in order to converse with each other, but I think no two people see "things" exactly the same way and thus our differences in opinion of the way things truly are.
Yet for each of us, what we believe, truly defines our particular realities, and not only what we believe, but also what we think and feel.
That is why the "reality" of a deranged person is quite different than that of our own.