he must study rivers

So [the Prince] must never let his thought stray from military exercises, which he must pursue more vigorously in peace than in war. These exercises can be both physical and mental. As for the first, besides keeping his men well organized and trained, he must always be out hunting, so accustoming his body to hardships and also learning some practical geography: how the mountains slope, how the valleys open, how the plains spread out. He must study rivers and marshes; and in all this he should take great pains. Such knowledge is useful in two ways: first, if he obtains a clear understanding of local geography he will have a better understanding of how to organize his defense; and in addition his knowledge of and acquaintance with local conditions will make it easy for him to grasp the features of any new locality with which he may need to familiarize himself.

The Prince, p. 48, George Bull trans. (Penguin edition)

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I am not unaware that many have held and hold the opinion that events are controlled by fortune and by God in such a way that the prudence of men cannot modify them, indeed, that men have no influence whatsoever. Because of this, they would conclude that there is no point in sweating over things, but that one should submit to the rulings of chance. This opinion has been more widely held in our own times, because of the great changes and variations, beyond human imagining, which we have experienced and experience every day. Sometimes, when thinking of this, I myself have inclined to this same opinion. None the less, so as not to rule out our free will, I believe that it is probably true that fortune is the arbiter of half the things we do, leaving the other half or so to be controlled by ourselves. I compare fortune to one of those violent rivers which, when they are enraged, flood the plains, tear down trees and buildings, wash soil from one place to deposit it in another. Everyone flees before them, everybody yields to their impetus, there is no possibility of resistance. Yet although such is their nature, it does not follow that when they are flowing quietly one cannot take precautions, constructing dykes and embankments so that when the river is in flood they would keep to one channel or their impetus be less wild and dangerous. So it is with fortune. She shows her potency where there is no well-regulated power to resist her, and her impetus is felt where she knows there are no embankments and dykes built to restrain her.

-p.79 Bull trans. (Penguin edition)