Wandering Thoughts

How is it then, that amidst the abundance of books which have been lately published almost on all subjects, we should have none upon wandering thoughts? at least none that will at all satisfy a calm and serious mind? In order to do this in some degree, I purpose to inquire,

I. What are the several sorts of wandering thoughts?
II. What are the general occasions of them?
III. Which of them are sinful, and which not?
IV. Which of them we may expect and pray to be delivered from?