BOOK THIRD.
The third book has to do with the youth as he is between the ages of twelve and fifteen. At this time his strength is proportionately greatest, and this is the most important period in his life. It is the time for labor and study; not indeed for studies of all kinds, but for those whose necessity the student himself feels. The principle that ought to guide him now is that of utility. All the master's talent consists in leading him to discover what is really useful to him. Language and history offer him little that is interesting. He applies himself to studying natural phenomena, because they arouse his curiosity and afford him means of overcoming his difficulties. He makes his own instruments, and invents what apparatus he needs.
He does not depend upon another to direct him, but follows where his own good sense points the way. Robinson Crusoe on his island is his ideal, and this book furnishes the reading best suited to his age. He should have some manual occupation, as much on account of the uncertain future as for the sake of satisfying his own constant activity.
Side by side with the body the mind is developed by a taste for reflection, and is finally prepared for studies of a higher order. With this period childhood ends and youth begins.