“I imagine that a hero is a man who does what he can. The others do not do it.”

Source: Norwegian Wood

1850s, Address before the Wisconsin State Agricultural Society (1859)

Source: What Matters Most is How Well You Walk Through the Fire

Reagan reportedly displayed a plaque with this proverbial aphorism on his Oval Office desk (Michael Reagan, The New Reagan Revolution (2010), p. 177). Harry S. Truman is reported to have repeated versions of the aphorism on several occasions. This exact wording was in wide circulation in the 1960s, and the earliest known variant has been attributed to Benjamin Jowett (1817–1893).
Misattributed

Source: Drenai series, The King Beyond the Gate, Ch. 20
Context: "We irritated him, he told me. Why did he get himself killed for us?" "Because he was a hero. And that is what heroes do. You understand?"

Source: The Roving Mind (1983), Ch. 25
Context: How often people speak of art and science as though they were two entirely different things, with no interconnection. An artist is emotional, they think, and uses only his intuition; he sees all at once and has no need of reason. A scientist is cold, they think, and uses only his reason; he argues carefully step by step, and needs no imagination. That is all wrong. The true artist is quite rational as well as imaginative and knows what he is doing; if he does not, his art suffers. The true scientist is quite imaginative as well as rational, and sometimes leaps to solutions where reason can follow only slowly; if he does not, his science suffers.

Source: The Tales of Alvin Maker, Red Prophet (1988), Chapter 2.