“Jovons saw the kettle boil and cried out with the delighted voice of a child; Marshal too had seen the kettle boil and sat down silently to build an engine. 杰文斯看见壶开了，高兴得像个孩子似的叫了起来；马歇尔也看见壶开了，却悄悄地坐下来造了一部蒸汽机。”——英国经济学家 凯恩斯（John Maynard Keynes，British economist）