Gilbert Harman 
Skepticism and the Definition of Knowledge [PDF ebook] 

Originally published in 1990. This study argues that scepticism is an intelligible view and that the issue scepticism raises is whether or not certain sceptical hypotheses are as plausible as the ordinary views we accept. It discusses psychological concepts, definitions of knowledge, belief and hypothetic inference (inference to the best explanation). Starting from ‘Is skepticism a problem for epistemology’, the book takes us through the argument for the possibility of scepticism, including looking at sense data and considering memory and perception.