“If Shakespeare required a word and had not met it in civilised discourse, he unhesitatingly made it up.”

Great Books: The Foundation of a Liberal Education (1954)

As quoted in Making a Habit of Success: How to Make a Habit of Succeeding, How to Win With High Self-Esteem (1999) by MacK R. Douglas, p. 45. Unsourced variant: What this power is, I cannot say. All I know is that it exists... and it becomes available only when you are in that state of mind in which you know exactly what you want...and are fully determined not to quit until you get it.
Context: I had made up my mind to find that for which I was searching even if it required the remainder of my life. After innumerable failures I finally uncovered the principle for which I was searching, and I was astounded at its simplicity. I was still more astounded to discover the principle I had revealed not only beneficial in the construction of a mechanical hearing aid but it served as well as means of sending the sound of the voice over a wire. Another discovery which came out of my investigation was the fact that when a man gives his order to produce a definite result and stands by that order it seems to have the effect of giving him what might be termed a second sight which enables him to see right through ordinary problems. What this power is I cannot say; all I know is that it exists and it becomes available only when a man is in that state of mind in which he knows exactly what he wants and is fully determined not to quit until he finds it.

Chap.IX: The Primitive and the Technical
The Revolt of the Masses (1929)

Songmaster (1979)

Sorrows of Werther, reported in Bartlett's Familiar Quotations, 10th ed. (1919).

Notes, p. 262.
The Lost World of Thomas Jefferson (1948)

Attributed in Lincoln the Lawyer (1906) by Frederick Trevor Hill — Hill noted that he could find no record of whom Lincoln was insulting.
Posthumous attributions

Mohican Press interview (2005)

Source: The Tales of Alvin Maker, Alvin Journeyman (1995), Chapter 10.