They don’t understand that a true captain must pay attention to the seasons of the year, the sky, the stars, the winds, and all that pertains to his craft, if he’s really to be the ruler of a ship. And they don’t believe that there is any craft that would enable him to determine how he should steer the ship, whether the others want him to or not, or any possibility of mastering this alleged craft or of practicing it at the same time as the craft of navigation. Don’t you think that the true captain will be called a real stargazer, a babbler, and a good-for-nothing by those who sail in ships governed in that way?

After Socrates presents his notion of a philosopher-king in Book 6, Adeimantus objects by pointing out that all real-life philosophers are either vicious or useless. Socrates responds by drawing an analogy to a ship governed by violent men, ignorant of navigation. His intention is to demonstrate that a good philosopher would necessarily be considered useless under current circumstances. True knowledge is not valued in modern Athens, nor even believed possible, and so anyone who tries to live their life by pursuing and praising real knowledge (as the true philosopher must do) will be thought a useless fool.