a mind could exist by observing that person's behaviour.

To be fully justified in believing a proposition to be true one must be acquainted, not only with the fact that supposedly makes the proposition true, but with the relation of correspondence that holds between the proposition and the fact. In other words, justified true belief can only occur if I know that a proposition (e.g. "Snow is white") is true in virtue of a fact (e.g. that the frequency of the light reflected off the snow causes the human eye, and by extension, the human