In Political Ideals (1917), Bertrand Russell wrote
The whole realm of thought and opinion is utterly unsuited to public control; it ought to be as free, and as spontaneous as is possible to those who know what others have believed. The state is justified in insisting that children shall be educated, but it is not justified in forcing their education to proceed on a uniform plan and to be directed to the production of a dead level of glib uniformity. Education, and the life of the mind generally, is a matter in which individual initiative is the chief thing needed; the function of the state should begin and end with insistence on some kind of education, and, if possible, a kind which promotes mental individualism, not a kind which happens to conform to the prejudices of government officials.
I've quoted a whole paragraph since I thought the subject may interest you. But I'm writing this post because I don't understand just the emboldened part: What does that mean in general? And who are "those who know what others have believed"?