The proper business of writing is to stay put on the page so that we can look at it later. Writing, whether it be a grocery list or The Brothers Karamazov , freezes the work of the mind into a permanent and public form. It is the mind and memory of mankind in such a form that we can pass it around to one another and even hand it on to our unimaginably remote descendants. Language is, essentially, speech. Writing is a special case of language.
Discursive prose is a special case of writing.
Written, discursive prose may be almost three thousand years old, but it is still our most recently invented use of language. It is no coincidence that the Greeks who first devised discursive prose also constructed formal logic and were the first to provide for their unimaginably remote descendants a visible record of the works of their minds. Thinking is coherent discourse, and the logic and the prose require one another. The mind is a rudderless wanderer blown here or there by any puff of breeze. The very rare genius can keep his mind on course for a while, perhaps as long as a whole minute, but most of us are always at the mercy of every random suggestion of environment.