Though Machiavelli usually refrains from using metaphors in his writing, there are three instances in The Prince were metaphors are used to support a point.

The first metaphor comes in chapter three.  Here, Machiavelli uses medicine to symbolize the prince's preparedness for unforeseen troubles.  If the prince takes the medicine early, before the problem is noticed, then he will be able to defeat the sickness when it comes.  But if he waits to take the medicine until the sickness shows itself, he will not have enough time to be cured.

The other metaphors occur much later, in chapter 18.  Here, Machiavelli describes the dual nature of humankind.  A good prince, he argues, must take on different forms to best fit his situation.  The prince must be able to use both the nature of humans and that of beasts.  On the animal side, he must be crafty like a fox and fearsome like a lion.  Machiavelli explains, "it is necessary to be a fox to discover the snares and a lion to terrify the wolves." The lion embraces religion and piety to frighten his enemies but the fox is clever enough to realize that religion is not always the answer and may actually backfire.  Though the face of the lion is best to show the public, being the fox is often necessary if the prince wants to be successful.