Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.

James Burgh, in The Dignity of Human Nature, Or, A Brief Account of the Certain and Established Means for Attaining the True End of Our Existence (1754); this is very widely misattributed to Mann, appearing at least as early as the publication of Thoughts Selected from the Writings of Horace Mann (1867) edited by Mary Mann.