The speaker of Sonnet 147 goes on and on in this sonnet about how his lust has caused him to lose his mind and his ability to "reason" or, think rationally. At one point, the guy even begins to sound delirious, especially when he describes the way his thoughts have been racing and how he's been babbling a bunch of stuff that isn't true. But by the time we reach the end of the sonnet, we begin to wonder if our madman has really, truly lost his mind. Even though he claims that his desire has caused him to go insane, he's always pretty self-aware, which tells us that he's more in control than he lets on.
Even though the speaker claims he's lost his mind, this "madman" business is just an act—the dude knows exactly what he's saying throughout the entire sonnet.
The speaker talks about his ability to reason as if it's a person (a doctor, to be exact), because it allows him to pretend that he's not to blame for using poor judgment.