It is the nature of ambition to make men liars and cheats, to hide the truth in their breasts, and show, like jugglers, another thing in their mouths, to cut all friendships and enmities to the measure of their own interest, and to make a good countenance without the help of good will.

Sallust, Bellum Catilinae, X, 5. This particular translation of the original Latin is from the essay "On Liberty" by Abraham Cowley: "Sallust, therefore, who was well acquainted with them both and with many such-like gentlemen of his time, says, 'That it is the nature of ambition' (Ambitio multos mortales falsos fieri coegit, etc.) 'to make men liars and cheaters; to hide the truth in their breasts, and show, like jugglers, another thing in their mouths; to cut all friendships and enmities to the measure of their own interest, and to make a good countenance without the help of good will.'" The Wikiquote page for Sallust has the quote and a different translation.