If knowledge is defined as justified true belief, why is it necessary to include "justified" in that definition. If I have a belief that corresponds with an objective state of affairs, why doesn't that count as knowledge regardless of justification? In the Theaetetus, Socrates seems to consider it self-evident that if one forms a belief based on unreliable testimony, that belief is not knowledge even if it true. I don't see why this is the case. If a delusional person tells me it is going to rain tomorrow, and I form the belief (which happens to be true) that it is going to rain tomorrow, why would that not be considered knowledge? Especially if I can use that belief to successfully guide my activity in the world? One more clarification: I can understand why justification matters with respect to the psychological process of forming a belief. I am talking about the definition of knowledge, which is already presupposed to be true.