Ludwig Wittgenstein is one of the twentieth century’s most important philosophers. Amongst other things, he’s writing about doubt and belief – any doubt presupposes I believe in something.

The following quotation is from “Über gewisheit”

§125. If a blind man were to ask me “Have you got two hands?” I should not make sure by looking. If I were to have any doubt of it, then I don’t know why I should trust my eyes. For why shouldn’t I test my eyes by looking to find out whether I see my two hands? What is to be tested by what? (Who decides what stands fast?) And what does it mean to say that such and such stands fast?

(Source: ed. G. E. M. Anscombe and G. H. von Wright
Translated by Denis Paul and G. E. M. Anscombe Basil Blackwell, Oxford 1969-1975)