Not P but Moby-Dick (15)

[Mike Jing]

>From Chapter 37:

What I’ve dared, I’ve willed; and what I’ve willed, I’ll do! They think me
mad—Starbuck does; but I’m demoniac, I am madness maddened! That wild
madness that’s only calm to comprehend itself! The prophecy was that I
should be dismembered; and—Aye! I lost this leg. I now prophesy that I will
dismember my dismemberer.

What does "That wild madness that’s only calm to comprehend itself" mean?