Difficulties strengthen the mind, as labor does the body.

困难锻炼心智，如同劳动锻炼身体。

underlie  verb

[ˌʌndərˈlaɪ] 

be or form the base for

构成…的基础；作为…的原因

例句：

In it you emphasise that rationality and calculation underlie economic behaviour. 

你在书中强调，理性和计算是经济行为的基础。