米塞斯认为边际效用递减律是“验前”（a priori）成立的。（见Human Action）我们暂且不对他的方法论作出评价。我关心的是，这是否是一个合法的推论？
下面一段文字是Hans-Herman Hoppe在他的Economic Science and the Austrian Method中作为例子提出来的。
Whenever the supply of a good increases by one additional unit, provided each unit is regarded as of equal serviceability by a person, the value attached to this unit must decrease. For this additional unit can only be employed as a means for the attainment of a goal that is considered less valuable than the least valued goal satisfied by a unit of such good if the supply were one unit shorter.
米塞斯和Hoppe的论证，如果我没有理解错的话，是这样：在任何时候，人们总是先满足他的最迫切的需要，然后才考虑“次优”的选择。因此，如果他吃掉了两个苹果，那就表明第二个苹果的效用比第一个低。因为不然，他就可以先吃第二个！
我想起一个笑话：一个人吃了6个饼（我不知道是几个，暂且当他是6个吧），吃第7个的时候，吃了一半就饱了，于是他很懊恼，说：为什么我不先吃第7个呢？
可是，这真是米塞斯的观点吗？也许我弄错了。这是我迟迟不敢提出这个问题的原因。米塞斯不会这么荒唐吧？不管怎么样，这直接违背了他的另一个原则：即，不同时刻的效用是无法比较的。因为我们不能在今天的苹果和明天的梨子之间进行选择。选择集里面的元素应该是不相容的，选择一个就意味着放弃了其他。而且，“明天的梨子”并不是“梨子”，而是“明天的梨子”。我们当然可以在“现在吃苹果”和“三分钟后吃苹果”之间进行选择，但比较的结果并不是“现在的苹果”和“三分钟之后的苹果”之间的排序，而是“现在吃苹果”这件事和“三分钟后吃苹果”这件事之间的排序。
那么，所谓的“边际效用递减”，到底是什么意思呢？在我看来，唯一有意义的表述应该是：边际替代率递减——即，环境不变（面临同样的选择集），某个事件的排序下降，我们就称为边际替代率递减。——如果世界上还有“边际效用递减”的话，我认为这就是“边际替代率递减”。比如你吃了两个苹果，当你吃第一个的时候，你认为苹果比梨子好，当你吃第二个的时候，你认为梨子比苹果好（但是你没有梨子，换句话说，就是你付不起梨子的价格），——这个时候，我们就说，第二个苹果的效用比第一个低，因此满足“边际替代率递减”或“边际效用递减”。除此之外，——因为任何直接的（跨时）比较都是不可能的，——说“边际效用递减”就是没有意义的。
如果这个分析成立，那么，“边际效用递减”就不是一个“验前”的判断。因为我们不能从“行动”的概念中推出这个命题。以吸毒为例（或者任何你能想到的例子，爱情什么的……），当一个人比以前更迫切地需要毒品时，当他“愿意”为毒品支付更多的钱时（其他条件不变），我们就可以推断毒品对他是“效用递增”的。但是当我们缩短时间跨度，比如在一天之内，我们可以设想，他不会二十四小时不间断地吸毒，他还要作别的事，他可能暂时觉得满足了，就去喝杯咖啡什么的，这时，毒品是“边际效用递减”的。
跨时的效用无法作直接的比较，但是可以进行间接的比较。暂时，这是一个可以接受，但并不那么严格的结论。因为我们必须假定，不同时刻的行为主体的效用函数不会发生“整体性的”变化，——如果全然不变，那就不会发生“边际效用递减”或“递增”这样的事情（递减或递增的根源仍然是效用函数的变化，所谓“稳定的效用函数”，那要看什么“问题”而定了）；但是如果整个变了，那就没什么好比较了（不同的人之间，无法比较）。
我不知道米塞斯的结论是怎么回事。我不相信他会这么粗心大意。但如果这是一个错误（错误的推理），那么这个错误太直接、太明显了。让人无法相信。
摘自Human Action第7章第1节：
The law of marginal utility and decreasing marginal value is independent of Gossen's law of the saturation of wants (first law of Gossen). In treating marginal utility we deal neither with sensuous enjoyment nor with saturation and satiety. We do not transcend the sphere of praxeological reasoning in establishing the following definition: We call that employment of a unit of a homogeneous supply which a man makes if his supply is n units, but would not make if, other things being equal, his supply were only n-1 units, the least urgent employment or the marginal employment, and the utility derived from it marginal utility. In order to attain this knowledge we do not need any physiological or psychological experience, knowledge, or reasoning. It follows necessarily from our assumptions that people act (choose) and that in the first case acting man has n units of a homogeneous supply and in the second case n-1 units. Under these conditions no other result is thinkable. Our statement is formal and aprioristic and does not depend on any experience.
There are only two alternatives. Either there are or there are not intermediate stages between the felt uneasiness which impels a man to act and the state in which there can no longer be any action (be it because the state of perfect satisfaction is reached or because man is incapable of any further improvement in his conditions). In the second case there could be only one action; as soon as this action is consummated, a state would be reached in which no further action is possible. This is manifestly incompatible with our assumption that there is action; this case no longer implies the general conditions presupposed in the category of action. Only the first case remains. But then there are various degrees in the asymptotic approach to the state in which there can no longer be any action. Thus the law of marginal utility is already implied in the category of action. It is nothing else than the reverse of the statement that what satisfies more is preferred to what gives smaller satisfaction. If the supply available increases from n-1 units to n units, the increment can be employed only for the removal of a want which is less urgent or less painful than the least urgent or least painful among all those wants which could be removed by means of the supply n-1.
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