Knowledge and Reasons for Belief
Suppose I believe that you plagiarized me. Suppose that it’s true that you plagiarized me. Suppose also that I have irrefutable evidence that you plagiarized me, in the form of my exact words appearing in a paper of yours (this evidence exists precisely because you plagiarized me). So I have a true justified belief that you plagiarized me, where my justification is not accidentally connected to the truth of what I believe. Do I then know that you plagiarized me?
What if I have always hated you and feel a strong sense of professional rivalry with you? I will believe anything against you no matter how flimsy the evidence may be. I would jump at the chance to accuse you of plagiarizing me. However, I am not consciously aware of these negative feelings and dispositions; I believe that my animus towards you is based purely on objective facts (you plagiarized me!). Suppose that the reason (cause) I believe that you plagiarized me is this unconscious animus; I would have believed it even in the absence of the evidence I have. That evidence rationally justifies my belief, but it is not the reason I have that belief. In fact, I care little about whether my belief concerning you is true—what I care about is that it makes me feel good to hold negative beliefs in your regard. The evidence I have is convenient for convincing other people of your culpability, but it plays no role in the formation of my belief (in fact the evidence is not as conclusive as I could wish, since you might have just forgotten where you got the ideas in question). My belief is caused by my unconscious attitudes and not by the objective evidence. There are good rational reasons for my belief and I am in possession of them, but they are not the operative reasons for me. My reasons are actually bad reasons for belief. I am forming my beliefs about you in a defective manner; I fall well short of epistemic responsibility.
It seems to me that in these circumstances I don’t know that you plagiarized me. My belief is true and justified, and the justification is properly connected to the truth, but still I don’t know. Anyone else in possession of the evidence I have would know, but I don’t know. They would have their belief shaped by the evidence, rationally so, but not me: the reason for their belief would be the evidence, but in my case that is not the reason for my belief. In order for me to know the evidence would have to be the reason for my belief. Thus the following conditions are necessary conditions for knowledge: the proposition must be true, you must believe it, you must be justified in believing it, and the justification must be the reason you believe it.  Simply put, the justification must cause the belief; alternatively, you must form the belief rationally. Irrational belief formation is logically consistent with possession of a rational justification, and in such a case knowledge is not possessed. True, you plagiarized me, and I have eminently good reasons to believe that you did: but I don’t know it, because I didn’t form my belief responsibly and rationally. Knowledge is rational true justified belief. Psychology must recapitulate epistemology.