His Life of Cesare Borgia was published in 1670.

It enjoyed a considerable vogue, was translated into French, and has been the chief source from which many writers of fiction and some writers of "fact" have drawn for subsequent work to carry forward the ceaseless defamation of the Borgias.
History should be as inexorable as Divine Justice.

Before we admit facts, not only should we call for evidence and analyse it when it is forthcoming, but the very sources of such evidence should be examined, that, as far as possible, we may ascertain what degree of credit they deserve.

In the study of the history of the Borgias, we repeat, there has been too much acceptance without question, too much taking for granted of matters whose incredibility frequently touches and occasionally oversteps the confines of the impossible.
One man knew Cesare Borgia better, perhaps, than did any other contemporary, of the many who have left more or less valuable records; for the mind of that man was the acutest of its age, one of the acutest Italy and the world have ever known.

That man was Niccolo Macchiavelli, Secretary of State to the Signory of Florence.