Time.

Wright of Derby, A Philosopher giving a Lecture on the Orrery in which a Lamp is Put in Place of the Sun, 1766

Hold childhood in reverence, and do not be in any hurry to judge it for good or ill. Leave exceptional cases to show themselves, let their qualities be tested and confirmed, before special methods are adopted. Give nature time to work before you take over her business, lest you interfere with her dealings. You assert that you know the value of time and are afraid to waste it. You fail to perceive that it is a greater waste of time to use it ill than to do nothing, and that a child ill taught is further from virtue than a child who has learnt nothing at all. You are afraid to see him spending his early years doing nothing. What! is it nothing to be happy, nothing to run and jump all day? He will never be so busy again all his life long. Plato, in his Republic, which is considered so stern, teaches the children only through festivals, games, songs, and amusements. It seems as if he had accomplished his purpose when he had taught them to be happy; and Seneca, speaking of the Roman lads in olden days, says, "They were always on their feet, they were never taught anything which kept them sitting." Were they any the worse for it in manhood? Do not be afraid, therefore, of this so-called idleness. 

Jean-Jacques Rousseau