 It is in this context that Russell also introduces his famous distinction between two kinds of knowledge of truths: that which is direct, intuitive, certain and infallible, and that which is indirect, derivative, uncertain and open to error (1905, 41f; 1911, 1912, and 1914b). To be justified, every indirect knowledge claim must be capable of being derived from more fundamental, direct or intuitive knowledge claims. The kinds of truths that are capable of being known directly include truths about immediate facts of sensation and truths of logic. Examples are discussed in The Problems of Philosophy (1912a) where Russell states that propositions with the highest degree of self-evidence (what he here calls “intuitive knowledge”) include “those which merely state what is given in sense, and also certain abstract logical and arithmetical principles, and (though with less certainty) some ethical propositions” (1912a, 109).