Macbeth:Macbeth Act 1, scene 7. 25–28
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself,
And falls on th'other. . . .
Macbeth, trying to rationalize his impending murder of King Duncan, continues his great "If it were done" soliloquy [see THE BE-ALL AND THE END-ALL]. Unfortunately, as Macbeth has just explained to himself, there's no real justification for the crime—Duncan is his relative, a meek and pious man, a good king, and, furthermore, a guest at his castle. All this argues against so bloody a deed, which will appear unjustifiable to mortal and divine eyes alike.
Therefore, Macbeth has no "spur" to prick on his intent, which is likened to a wild steed—no motivation to inspire the murder. Continuing the horse metaphor, he can only draw on "vaulting ambition": an intense desire for power. His desire vaults even beyond its intrinsic limits ("o'erleaps itself") to land on "th'other" (the other side)—probably, to land somewhere unknown and beyond reason.
Compare this speech to its parallel in Julius Caesar—Brutus's soliloquy in which he rationalizes an assassination, but comes up with more probable motivations than Macbeth's [see THE SERPENT'S EGG].