I dare do all that may become a man,
Who dares do more, is none.
This topic, which has been always employed with too much success, is used in this scene with peculiar propriety, to a soldier by a woman. Courage is the distinguishing virtue of a soldier, and the reproach of cowardice cannot be borne by any man froma woman, without great impatience.
She then urges the oaths by which he had bound himself to murder Duncan, another art of sophistry by which men have sometimes deluded their consciences, and persuaded themselves that what would be criminal in others is virtuous in them; this argument Shakespeare, whose plan obliged him to make Macbeth yield, has not confuted, though he might easily have shown that a former obligation could not be vacated by a latter: that obligations laid on us by a higher power, could not be overruled by obligations which we lay upon ourselves.
Quoted in Arthur Sharbo. Samuel Johnson's Critical Opinions: A Reexamination. U of Delaware Press: Newark (1995), p. 161. (The speech to which Johnson is referring, and to which this is appended as a note, is in Macbeth I.vii.)