Sentence:  The severity of judgement, they say, makes men censorious and unapt to pardon the errors and infirmities of other men: and on the other side, celerity of fancy makes the thoughts less steady than is necessary to <WSD>discern</WSD> exactly between right and wrong. 
Meaning of the word marked with <WSD> tags:  To distinguish something as being different from something else; to differentiate.