Article 2: Opportunity cost

孟子說：「魚與熊掌，皆我所欲也。兩者不可兼的，捨魚而取熊掌也。」可見，有選擇，就有取捨。選了認為是最佳的項目，選了認為是最佳的項目，捨掉的次佳項目就是成本。孟子決定選擇熊掌時，他的機會成本就是捨掉了的魚。

Consider the house cleaning example. At 9:00 a.m., when John is about to do the cleaning, he still has the option of going on the picnic. His cost of cleaning the house is the giving up of the value of the picnic. The cleaning is finished by 5:00 p.m. By then, is there still a cost of cleaning the house?

By 5:00 p.m., any value from the picnic has already been given up. The cost of cleaning the house is a past cost. As John no longer has the option of going on the picnic, we cannot say that there is still a cost of cleaning the house. Past costs are not costs because they do not represent or future option to be given up.