The English language dimly recognizes that knowing something is not the same as believing it sure to be true. Ptolemy believed it sure that the sun revolved around the earth, but we would not say that he knew the sun revolved around the earth. Philosophers commonly claim that the difference lies in the fact that knowledge should be regarded as justified true belief, but such an attempt at a definition has had little influence in rational choice theory, presumably because the question of what should count as a justifying argument is left hanging in the air.