For 2,000 years, the standard philosophical model of knowledge was that it could be defined as a justified true belief. According to this way of thinking, we can know, for example, that we are human because [1] we believe ourselves to be human; [2] that belief is justified (others treat us as humans, not as dogs); and [3] the belief is true. This definition, which dates to Plato, was challenged by Edmund Gettier in one of the most influential works of philosophy published in the last century – a three page paper that produced two clear examples of justified true beliefs that could not,...