Popper believed that objective knowledge comes about by writing ideas down:
Putting your ideas into words, or better, writing them down makes an important difference. For in this way they become criticisable, Before this, they were part of ourselves. We may have had doubts. But we could not criticize them in the way in which we criticize a linguistically formulated proposition or, better still, a written report. Thus there is at least one important sense of "knowledge"—the sense of "linguistically formulated theories submitted to criticism." It is what I call "knowledge in the objective sense". Scientific knowledge belongs to it. It is knowledge which is stored in our libraries rather than in our heads.Knowledge stored in libraries and in our notes is much more important than knowledge stored in our heads; it leads to the growth of knowledge in the objective sense of the word because it allows of and enables serious criticism. That is one of the reason why I consider it so important to take notes.
1. See Bryan Magee, Modern British Philosophy. Dialogues with A.J. Ayers, Stuart Hampshire, Alisdair MacIntyre, Alan Montefiore, David Pears, Karl Popper, Anthony Quinton, Gilbert Ryle, Ninian Smart, Peter Strawson, Geoffrey Warnock, Bernard Williams, Richard Wollheim. (New York: St. Martin's Press, 1971), p. 74.