Scepticism, closure and rationally grounded knowledge: a new solution

Abstract

Radical scepticism contends that our knowledge of the external world is impossible. Particularly, radical scepticism can be motivated by the closure principle (i.e., if one knows that P and one can competently deduce Q from P, then one is in a position to know that Q). Several commentators have noted that a straightforward way to respond to such arguments is via externalist strategies, e.g., Goldman (Justification and knowledge, Reidel, Dordrecht, 1979, Epistemology and cognition, Harvard University Press, Cambridge, 1986), Greco (Putting skeptics in their places, Cambridge University Press, New York, 2000), Bergmann (Justification without awareness, Oxford University Press, New York, 2006). However, these externalist strategies are not effective against a slightly weaker form of the argument, a closure principle for rationally grounded knowledge, \({closure}_{RK.}\) The sceptical argument, framed around the \(\hbox {closure}_{\mathrm{RK}}\) principle (i.e., if S has rationally grounded knowledge that P, and S competently deduces Q from P, thereby forming a belief that Q on this basis while retaining her rationally grounded knowledge that P, then S has rationally grounded knowledge that Q), targets rationally grounded knowledge. Although externalist strategies are ineffective against this form of argument, its conclusion can nonetheless be resisted by combining, in a novel way, the resources of Wittgenstein and Davidson. In particular, I argue that the sceptic is assuming an unrestricted way of using the \(\hbox {closure}_{\mathrm{RK}}\) principle, which is incorrect. Alternatively, I argue for The Conditionality of Rational Support Thesis, i.e., the thesis that rational support via competent deduction is conditional. In particular, rational support must be provided within an evaluative system in which hinge propositions are presupposed and contentful beliefs are being evaluated. In the end, we can resist the \(\hbox {closure}_{\mathrm{RK }}\)-based sceptical argument while retaining the \(\hbox {closure}_{\mathrm{RK}}\) principle.