The act of writing is itself an exercise of thought
Many writers have said that they do not fully grasp their own meaning until they have carved it like a statue, using words as material. The reason is plain. One starts writing, not with a well-shaped thought, trimmed and polished, but with an intent—perhaps with several, overlapping and conflicting. You see a scene in your mind’s eye or know the tendency of a complex argument, but do not know which part of the scene or argument is to come first—what, anyhow, is a part of something you sense as an undivided whole? Thinking, and nothing but thinking, will answer these questions; nor will the answer be satisfactory until words are down on paper that represent the first finished piece of description or argument. (p.118-9).
Jacques Barzun. Simple & Direct