A dispute in epistemology has arisen over whether some class of things epistemic (things known or justified, for example) is closed under some operation involving the notion of what follows deductively from members of this class. Very few philosophers these days believe that if you know that p, and p entails q, then you know that q. But many philosophers think that something weaker holds, for instance that if you know that p, and p entails q, then you are in (...) a position to know that p, or if you know that p and you competently deduce p from q, then you know that q. However there are some considerations tracing back to Wittgenstein’s On Certainty and some early work by Fred Dretske that suggests even these weaker principles are false. (shrink)

The principle that epistemic justification is necessarily transmitted to all the known logical consequences of a justified belief continues to attract critical attention. That attention is not misplaced. If the Transmission Principle is valid, anyone who thinks that a given belief is justified must defend the view that every known consequence of the belief is also justification of the conclusion in an obviously valid argument. Once created, the gap is hard to fill, whatever the circumstances. Reflection principle is modified, the (...) possibility of deductive justification is threatened. If some known consequence fails to be justified, the failure may extend to every known consequence. To reject Transmission is to insert a logical gap between the justification of a premise and the justification of the conclusion in an obviously valid argument. Once created, the gap is hard to fill, whatever the circumstances. Reflection on the Transmission Principle therefore usefully brings us face to face with the following dilemma: accept the principle and hand Cartesian scepticism a powerful weapon, or modify the principle and risk undermining deductive justification. (shrink)