Knowledge differs from belief in that the one is a conception which is invariable and always the same while the other is a conception which can be otherwise.
For example, as for Socrates both knowledge and belief assume that he is a man; and in this sense they are both true. But while knowledge says that Socrates is essentially a man – this is the same as to say that he is not a not-man, which means it is impossible not to be a man – belief as if it may too not to be a man. In this way, then, they differ in virtue of the way of the conception.
It is clear from what has been said that it is impossible for one to opine and know well the same thing simultaneously; for then one would assume that the same thing is at the same time both capable and incapable of being otherwise – which is not possible. For two different people each of those, or at another time, if one and the same, such a case could be possible; however, with respect to the same time it is impossible for numerically one and the same man to opine and know well the same thing simultaneously. For he assumes that it is not capable of being otherwise in so far as he has a good knowledge of the thing, but inasmuch as he has a belief he assumes that it can be otherwise.
Bibliography: Aristotle’s Posterior Analytics, J. Philoponus in Analytics
Translation – text editing: George Kotsalis