Solitude is the climax of the negative virtues. When we go to bed after a solitary day we can tell ourselves that we have not been unkind nor dishonest nor untruthful; and the negative virtues are agreeable to that dangerous faculty we call the conscience. That they should ever be admitted for a part of virtue is what I cannot explain. I do not care two straws for all the nots.

Robert Louis Stevenson (1850–1894) Scottish essayist, novelist, poet

Essay (1880-01/02?), “Reflections and Remarks on Human Life,” § 4.6 “Solitude and Society”
    (Source)

A collection of aphorisms and musings, first published in the Edinburgh Edition of his Works, vol. 28 (1898).