Putting Ideas into Words
Writing about something, even if you know it well, often reveals gaps in your understanding. Putting ideas into words is a severe test. The initial words chosen are usually wrong, requiring multiple rewrites. Your ideas will not only be imprecise but incomplete. Half of the ideas in an essay emerge during the writing process. Once published, it seems those were your original thoughts, but the act of writing changes them.
Reading what you've written as a neutral reader is the real test. You must assess if it seems correct and complete. This process often reveals deficiencies that need addressing. The "stranger" reader is rational and can be satisfied if you meet their needs. This may cost you some nice sentences, but it's necessary for clarity.
Few people can perfectly translate thoughts into words. In precisely defined domains like chess or math, complete ideas can be formed in the head. However, these are expressed in formal languages. Writing essays in your head is possible to some extent, but it is still writing.
Writing about a subject you know well often reveals unconscious knowledge. Experts, in particular, have a higher proportion of unconscious knowledge. Writing is not the best way to explore all ideas, but it always yields new insights.
Writing demands a single, optimal sequence of words, unlike talking. It allows for a level of focus that conversation cannot match. If you're lazy, both writing and talking are useless, but writing is the steeper hill for those seeking precision.
No one has fully formed ideas about a topic unless they've written about it. Ideas can feel complete, but writing reveals their true state. Writing is a necessary condition for fully formed ideas, even if not a sufficient one.
The original article: http://paulgraham.com/words.html