What is knowledge? That is, under what conditions can a person truly be said to know that something is true? You might that that a person knows that some claim, P, is true when (and only when) (1) that person believes that P is true, (2) that person is justified in believing that P is true, and (3) P is in fact true.
However, the philosopher Edmund Gettier (1927–) made famous counter-examples to this offered answer. Take, for consideration, the (embarrassingly) true story in this poem. I once had the belief that my twin sister was nearby. Now, I was justified in believing she was nearby because I saw an image of her, and having an apparent perception of something is quite a reliable way of coming to know that something is there. It was also true that my sister was nearby. Nonetheless, I did not know that my sister was there, because I was in fact looking (and waving) at my own reflection, and it was merely a coincidence that she was in the vicinity! | |||