In epistemology, Wright has argued that G. E. Moore's proof of an external world ("Here is one hand") is logically valid but cannot transmit warrant from its premise to the conclusion, as it instantiates a form of epistemic circularity called by him "warrant transmission failure". Wright has also developed a variant of Ludwig Wittgenstein's hinge epistemology, introduced in Wittgenstein's On Certainty as a response to radical skepticism. According to hinge epistemology, there are assumptions or presuppositions of any enquiry – called "hinge propositions" – that cannot themselves be rationally doubted, challenged, established or defended. Wright contends that certain hinge propositions can actually be rationally held because there exists a type of non-evidential, "a priori" warrant – which Wright calls "epistemic entitlement" – for accepting them as true.