Finished editing this, then forgot to post it. (sigh)
"Eugenics and Other Evils," by G.K. Chesterton, published in 1922.
Chapter 4 -- The Lunatic and the Law:
"The modern evil, we have said, greatly turns on this: that people do not see that the exception proves the rule. Thus it may or may not be right to kill a murderer; but it can only conceivably be right to kill a murderer because it is wrong to kill a man."
Comments to 218-214-CALL (218-234-2255)