The problem for rationality is to decide whether classes are real (ie, can be found) or our inventions, to reach a consistent rational understanding of reality. This problem is, however, just a warm-up for the next problem: that the former entrance ends in paradoxical contradiction, whereas the latter ends in ambiguity.
The insurmountable problem to reach a consistent understanding of reality is thus that reality in rational understanding is either paradoxically contradictory or ambiguous, or both. It dwells in the incomprehensible black hole between paradoxical contradiction and ambiguity. The truth is thus that we can’t reach a consistent understanding of reality. (So, don’t believe in a “Higgs particle” or a “tree of life”, both of these are faiths rather than rationality).