It is not a matter of theory but of facts.

Millions of men have in the past acquired the faculty of being able to repeat and remember whatever they heard, if they earnestly attended to it.

Earnest attention in this case means a strong exercise of forethought, or determination to an end or given purpose.

In Iceland, that which has since become the English common law, was at an early date very fully developed, without any books or writing.

And there were lawyers who had by heart all the laws, and incredible numbers of precedents, as appears from several sagas, among others, that of The Burnt Njall.
Our present system of Education is that of building houses without foundations.