5 ideas
Outcomes are a poor test of a counsel's quality: a well-reasoned plan can fail through fortune, and a reckless one can succeed by luck. Praising or blaming advisers by results teaches the wrong lessons, so judgment must be passed on what was knowable and weighed at the moment of choice.
During the 1529–30 siege of Florence by imperial and papal forces, experienced observers expected the city to fall within days. Guicciardini cites this as evidence that conviction produces effects that cool calculation fails to foresee, and that confident experts misjudge what committed people can endure.
Rules and maxims fail in practice because small differences between cases change what the right action is, and these differences are rarely written in any book. Reasoning by historical example, such as citing the Romans at every turn, is misleading unless conditions truly match. The deciding faculty is discrezione: a trained eye for the particulars of each case.
A person's loyalties follow their particulare, their personal stake, more than their stated convictions. Guicciardini admits that he despised clerical corruption and would have favored Luther, yet his career in papal service bound him to defend the popes' power. To predict what someone will do, look at where their advantage lies, including your own.
The future is too uncertain for anyone who claims to predict it to be trusted, and waiting for certainty means never acting. The prudent course is to weigh the probabilities, commit to the side where the balance leans, and use delay when time is likely to reveal more. Then accept calmly that fortune may still overturn a sound choice.