To say that I know that something is the case, though it does imply that I am sure of it, is not so much to report my state of mind as to vouch for the truth of whatever it may be. In saying that I know it I engage myself to answer for its truth: and I let it be understood that I am in a position to give this undertaking. If my credentials do not meet the usual standards, you have the right to reproach me. You have no right to reproach me if I merely say that I believe, though you may think the less of me if my belief appears to you irrational. If I tell you that I believe something which I do not, I am misinforming you only about my mental attitude; but if I tell you that I know something which I do not, the chances are that I am misinforming you about the truth of the statement which I claim to know, or if not about its truth, then about my authority to making it.