“Ever since the time of Plato, philosophers have tried to offer an adequate definition of propositional knowledge (hereafter, simply called knowledge). In his dialogue Theaetetus, Plato offered (though he did not completely endorse) what is known as the standard definition of (propositional) knowledge. The standard way of stating this definition is to say that knowledge is justified true belief (sometimes called the tripartite analysis or alluded to as simply JTB). It will be helpful to analyze this definition further. If someone knows something, then what he knows must be true. It would make no sense to say that Jones knows that milk is in the refrigerator but that, nevertheless, it is false that the milk is there. So a necessary condition of knowledge is that what is known is true. But truth is not sufficient for knowledge. There are many truths that no one has ever thought of, much less known. And there are some truths that someone may think about but not know.
Besides truth, a second part of knowledge is belief. If Jones knows something in the propositional sense, he must at least believe it. It would make no sense to say that Jones knows that milk is in the refrigerator but that, nevertheless, he does not believe that milk is in the refrigerator. So belief is a necessary condition for knowledge. But mere belief is not sufficient for knowledge. People believe many things that they do not know to be true.
True belief is a necessary condition for knowledge. But is true belief sufficient for knowledge? No, it is not. The reason is that someone can believe things that are true but have no justification or warrant at all for those beliefs. It may be that one’s belief is true by simple accident… For present purposes, the main idea is that there is a big difference between a mere true belief and a true belief that has warrant or justification. And the traditional or standard definition of propositional knowledge is the view that knowledge is justified true belief.” – Excerpts from Philosophical Foundations for a Christian Worldview by J.P. Moreland and William Lane Craig