We imagine that we sit down and think, but, in fact, we mostly gather wool, remembering this and that and fantasizing about the other. In our heads we recite some slogans and rehash the past, often repeatedly. Even in this foolish maundering, we are easily distracted by random thoughts, mostly about money or politics but often about sports or sex. Left to its own devices, the mind plays like a child in well-stocked sandbox, toying idly with trinkets and baubles and often doing the same thing over and over again until some slightly more interesting game presents itself. If we want to pursue extended logical thought, thought that can discover relationships and consequences and devise its own alternatives, we need a discipline imposed from outside of the mind itself.
Writing is that discipline.
It seems drastic, but we have to suspect that coherent, continuous thought is impossible for those who cannot construct coherent, continuous prose. Writing does indeed make us exact because it leaves a trail of thought that we can retrace and so discover where we have been stupid. In his writing, then, we can judge of at least two things in a man his ability to think and his intention to do so, his maturity. To understand the importance of writing for people who want to have a civilization, it is useful to compare discursive prose with poetry. Like speech, poetry is metaphorical and figurative, elliptical, often more expressive than informative, synthetic rather than analytic, and concrete rather than abstract.