John Derbyshire shares one of his favorite quotes from Dr. Johnson, recorded by Boswell:
Says the sage: “My dear friend, clear your mind of cant. You may talk as other people do: you may say to a man, ‘Sir, I am your most humble servant.’ You are not his most humble servant….You tell a man, ‘I am sorry you had such bad weather the last day of your journey, and were so much wet.’ You don’t care six-pence whether he was wet or dry. You may talk in this manner; it is a mode of talking in society: but don’t think foolishly.”
The italics in that opening commandment are absolutely necessary, as the rest make clear. Johnson did not object to people speaking insincerely for the purpose of lubricating social exchanges. Most of us agree with him on this, and the few who don’t are annoying and unpersuasive.
What’s happened there is that Johnson’s word “cant” has been mistakenly read as, or deliberately transformed into “can’t” with an apostrophe, making it motivational: “Clear your mind of CAN’T!”