LECHERY
I am one that loves an inch of raw mutton better than an ell of fried stock-fish, and the first letter of my name begins with lechery. (2.3.156-158)
Lechery, or lust, tells a dirty joke here. The "inch of raw mutton" likely refers to an erect penis, whereas "fried stockfish" (a textual error for "dried stockfish") is a slang expression that refers to sexual impotence. You get the idea…
FAUSTUS
These metaphysics of magicians
And necromantic books are heavenly.
Lines, circles, letters, and characters –
Ay, these are those that Faustus most desires. (1.1.48-51)
Describing "necromantic books" as "heavenly" is more than a little ironic considering the fact that these books will actually cause Faustus to turn away from heaven, and toward hell. Oh Faustus, don't you hear yourself?
FAUSTUS
Valdes, sweet Valdes, and Cornelius,
Know that your words have won me at the last
To practice magic and concealèd arts.
Philosophy is odious and obscure;
Both law and physic are for petty wits.
'Tis magic, magic that hath ravished me. (1.1.98-103)
We're just gonna address the elephant in the room: Faustus's creepy obsession with magic is almost erotic. He says that magic has "ravished" him, as though he were a maiden being taken by a powerful man. How could philosophy, law, or medicine hope to compete with an attraction like that?