Machiavelli uses fortuna to refer to all of those circumstances which human beings cannot control. While, Virtù is the human energy or action that stands in opposition to fortune. In Chapter 7 of The Prince, Machiavelli writes, “Only a brilliant and immensely skillful [di grande virtu] man is likely to know how to command without having training and experience.” He also says, “if those who are suddenly made into rulers and are of such extraordinary capacity [virtu] they can work out on the spot how to hold on to that gift fortune has unexpectedly handed them”. This is accomplished by building up resistance to fortuna through the use of Virtù, or the human energy or action that stands in opposition to fortune.
In The Prince Chapter. 7-8, Machiavelli also contrasts virtù with fortune in the sense of luck or the favor of powerful people. In those chapters, the contrast is between what the prince can control (his own actions) and what he cannot control (the favor of others).