 Another strand of skeptical thought begins with questions about the nature of philosophical investigation. In the Meno, Plato formulates a famous puzzle. How is investigation possible? We cannot investigate either what we know or what we do not know. In the former case, there is no need to investigate, and besides, if we really do know we already have in mind everything that investigation could reveal to us about the matter. In the latter case, we would not know what to look for, and we would not recognize it if we found it (80d-86c). So there is no room for investigating anything. Socrates calls this an eristic argument, thus drawing attention to the fact that this is a puzzle that sophists have put forward (cf. Plato’s Euthydemus).