"as the diseased otherwhile, that dream they drink of some clear spring, yet they are not satisfied, nor leave off so" (Castiglione 3).
"It shall even be as when an hungry man dreameth, and, behold, he eateth; but he awaketh, and his soul is empty: or as when a thirsty man dreameth, and, behold, he drinketh; but he awaketh, and, behold, he is faint, and his soul hath appetite" (Isaiah 29:8).
One of Castiglione's long-winded courtiers makes the case that the pursuit of lust is like drinking in a dream without being satisfied. He may here be drawing upon the words of Isaiah, who states that those who fight against Israel will be unsatisfied as those that eat or drink in dreams. One of these authors is a Renaissance man writing a courtesy book; the other is a Hebrew prophet speaking in the name of God.
At the risk of sounding like I'm trying to force an applicable moral out of two texts that are older than dirt, I think there's a common theme here--a warning from the voices of those who have watched many of their associates struggle a lifetime to gain something ephemeral, fleeting and ultimately worthless.
The people Isaiah speak of (I shall here pretend to have a clue as to what Isaiah is talking about, as if anyone does) are the figuratively assembled hosts battling against Zion: presumably (drawing on Isaiah's typical rhetoric) the unclean, the unrepentant, and the uncircumcised. I don't have much to say about the uncircumcised. However, the unclean and the unrepentant are archetypes that Isaiah usually paints as Ultimate Battle Prostitutes, simultaneously murdering the righteous on Mount Zion and engaging in every kind of loose sexual behavior.
Machiavellian Princes, perhaps?
Castiglione's discussion in the referenced chapter mostly concerns the question of whether it's okay for old men to pursue romance. I am moved to powerful indifference by this topic. But the mention of dissatisfied dream-drinking as a metaphor for the pursuit of lust brings to mind the common conception of court culture, with its associated backstabbing and sexual intrigue. Maybe both of these authors are speaking of the same group, even though is writing a condemnation of it and the other is writing a how-to manual for it. And if Castiglione is indeed aware of Isaiah, maybe their purposes are not as separate as they seem.
Castiglione, Baldassare. The Courtier. Vol. IV. N.p.: W. W. Norton, n.d. Web. Sep. 23 2015.