This is a short one. Popper said that we must:
…[be] consciously critical of [our] theories which, for this reason, [we should try] to formulate sharply rather than vaguely.
Only objective knowledge is criticizable: subjective knowledge [knowledge in the head] becomes criticizable only when it becomes objective. And it becomes objective when we say what we think; and even more so when we write it down, or print it.Objective Knowledge, p. 25)