Many problems in epistemology are of the "you can't get there from here" form; which is due to an assumption of justificationism.
Beliefs are seen as something like theorums which must be justified (proved) from other beliefs. And beliefs which can't be derived this way are illegitimate.
This, most notoriously, excludes us from knowledge of the real world. It's presumed that our belief formation is constrained by what we are. (Our biases, conceptual framework etc.) And because of this, there is a shortfall between the model constructed in our minds and the way the world really is.
Because it seems impossible that we could find some foundational knowledge of how the world is, independent of our phenomenally constrained subjective knowledge, it's presumed we have no way of justifying any talk of the shortfall between or comparison of the real and phenomenal world. Because of this we can't even refer to it.
However, from a CriticalRationalism perspective we can conjecture the idea of the real world and the gap between our phenomena and it. And therefore refer to this.
(This doesn't help us know that our beliefs about the real world are correct, but we can assert them. And, of course, subject them to attempts at improvement through criticism as with all our other ideas.)
See also ContinentalPhilosophy