your friend of a sovereign you have lent him, which he has forgotten to repay—you might quite mean the words "I mention it, as you seem to have a conveniently bad memory for debts", in jest: yet there would be nothing to wonder at if he took offence at that way of putting it. But, suppose you wrote "Long observation of your career, as a pickpocket and a burglar, has convinced me that my one lingering hope, for recovering that sovereign I lent you, is to say 'Pay up, or I'll summons yer!'" he would indeed be a matter-of-fact friend if he took that as seriously meant!
My eighth Rule. When you say, in your letter, "I enclose cheque for £5", or "I enclose John's letter for you to see", leave off writing for a moment—go and get the document referred to—and put it into the envelope. Otherwise, you are pretty certain