He is remembered in philosophy for the spirited essay ‘The Ethics of Belief’ (1879) defending an uncompromising religious agnosticism based on the evidentialist principle that ‘It is wrong, always, everywhere, and for anyone, to believe anything on insufficient evidence’. The essay brought an attempted rebuttal by William James, in his essay ‘The Will to Believe’ (1896), which itself, however, looks too much like a defence of wishful thinking to be convincing.

http://www-history.mcs.st-andrews.ac.uk/Biographies/Clifford.html A biography of Clifford, including his contributions to science, mathematics, and philosophy