It concentrates his mind wonderfully

I did not know this origin story to a famous Samuel Johnson quote. From A Samuel Johnson celebration recalls his wit and wisdom by Israel Shenker.

"In a man's letters", Johnson once wrote her [Hester Thrales], "his soul lies naked," and his compressed, elegant phrases, words unerringly targeted, sparkle through the intimate correspondence. In many compositions that were credited to others, Johnson, with unfailing charity, when moved by some great issue or petitioned by some worthy friends, contributed the bounty of his wisdom. "It is sufficient that our brother is an want," he counseled. "By which way he brought his want upon him, let us not too curiously enquire. We likewise are sinners."

When the Rev. William Dodd was sentenced to death for forgery, Johnson secretly wrote Dodd's appeal, and when that failed, went on to write his farewell. When someone later expressed doubts that Dodd could have composed prose so telling, Johnson responded: "Depend on it, Sir, when a man knows he is to be hanged in a fortnight, it concentrates his mind wonderfully.