Robin McKinley quote: “If you wish, I shall go personally to your City and knock together the heads of Perlith and Galooney.”

“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.”

Horace Mann (1796–1859) American politician

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.
Misattributed

“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 (1714–1775) British politician

Variant in other editions: Do not think of knocking out another person's brains, because he differs in opinion from you; it will be as rational to knock yourself on the head because you differ from what you thought ten years ago.
The Dignity of Human Nature (1754)

“"You disappear so completely into your head sometimes. I wish I could follow you."
You do. You live in my head all the time.”

Cassandra Clare book City of Ashes

Jace and Clary, pg. 354
Variant: You disappear so completely into your head sometimes," he said. "I wish I could follow you.
Source: The Mortal Instruments, City of Ashes (2008)

“But, very well, I believe I know you very well,
Wish that you knew me too, very well,
And I think I can deal with everything going through your head.”

Stevie Wonder (1950) American musician

Superwoman (Where Were You When I Needed You)
Song lyrics, Music of My Mind (1972)

“If you know that "I", in the sense of the person, the front, the ego, it really doesn't exist. Then…it won't go to your head too badly, if you wake up and discover that you're God.”

Alan Watts (1915–1973) British philosopher, writer and speaker

Alan Watts Teaches Meditation (1992)

“In Wing Chun if you throw two fast punches to someone's head they'll be knocked out. The first punch causes the brain to go to one side of the skull. If a quick second hit comes, the person is knocked out. If you withdraw the hand to give the second punch, then the brain can recover (will have more time to recover), but if you don't give this time then a knockout results.”

Wong Shun Leung (1935–1997) martial artist

Wong Shun Leung Comments on the Chain Punches in Wing Chun
Punching
Source: Comments From Wong Shun Leung and Tsui Shan Ting, by Ray Van Raamsdonk http://www.springtimesong.com/wcqanda.htm