Writing is a form of thinking

While reading about note taking philosophies, I came across a sentence which can be paraphrased as follows:

Writing is a form of thinking. When you write, you clarify your thoughts and make it concrete

I can relate to this. I have often said that the best way to learn a topic is to write about it, or teach it. It is a mistake to think that you first need to know a topic before you can write or teach about it. No, it is the writing and teaching that helps make you an expert, not the other way around.