Historically, since Plato, the most frequent definition of knowledge has been Justified True Belief (JTB). Until recently (the 20th century), philosophers believed in a foundation for JTB. The Cartesian Rationalists believed that some beliefs were certain because they were self-evident truths that survived Descartes method of doubt. The empiricists believed that some beliefs were self-evident truths because they were in agreement with observational experience. The Kantian Idealists believed that some beliefs were synthetic a prioris and as such were also certain.