I just obtained a copy of Mark Tredinnick’s book Writing Well (published down under as The Little Red Writing Book). I was struck with a quotation in his preface:

Writing is the most exact form of thinking.
(Carol Gelderman, All the Presidents’ Words)

I’ve found it increasingly helpful to clarify my own thinking by writing in response to what I read. Somehow Gelderman seems to have encapsulated the process exactly.