ABSTRACT
Marginalism became the ascendant notion of economics through its discovery that a principle of conduct which was natural and intuitive in men's management of their affairs, namely to pay attention to differences, to what would be the extra gain of a small extra sacrifice, was in fact the key idea of a formal mathematical procedure for determining that numerical value of the argument of a function, to which there corresponds an extreme value of the function. Intuitive conduct was logically correct and justified conduct, it was mathematically correct conduct, it was conduct which led to making the most of given resources, or getting given effects from the least resources, it was conduct which achieved economy. But this fact opened the tool-box of the differential and integral calculus for the economist's use. The 'subjective ideal' is the life governed by pure reason, able to be completely analysed and understood by the unaided logician.
