He had discovered a great law of human action, without knowing it -- namely, that in order to make a man or a boy covet a thing, it is only necessary to make the thing difficult to attain.
他发现了人类行为的一大法则，自己还不知道——那就是，为了要使一个大人或小孩极想干某样事情，只需要设法把那件事情弄得不易到手就行了。