 Another example of the interdefinability of modal concepts deals with the relationship between knowledge and belief. In  epistemology,  researchers are searching for the correct characterization of knowledge, and a common trend has been to view knowledge as a form of  justified true belief  (an idea that can be traced back to Plato’s dialogue Theaetetus). Gettier’s  famous counterexamples  showed that such a simple characterization of knowledge is not sufficient: a further condition is required, such as safety, sensitivity, robustness or stability. In spite of this, a characterization of knowledge as justified true belief is an important first step. Classic  epistemic logic  does not explicitly deal with the notion of  justification,[2]  so a starting point is a simpler understanding of knowledge as true belief.