I figured a contradiction in Kant's Universalizability principle, but I'm very surprised that it was so easy to prove that wrong, so I think that I might be wrong somewhere.

Let us first begin with a definition of his principle:

The concept of universalizability was set out by the 18th-century German philosopher Immanuel Kant as part of his work Groundwork of the Metaphysics of Morals. It is part of the first formulation of his categorical imperative, which states that the only morally acceptable maxims of our actions are those that could rationally be willed to be universal law. The precise meaning of universalizability is contentious, but the most common interpretation is that the categorical imperative asks whether the maxim of your action could become one that everyone could act upon in similar circumstances. If the action could be universalized (i.e., everyone could do it), then it is morally acceptable. Otherwise, it is not. (Wikipedia)

Here's my work:

  1. If everybody eats potato in one day, then the world would be out of potato.

  2. If the world is out of potato, then the action "eating potato" will not be possible.

  3. Then eating potato cannot be universalized.

  4. Then eating potato is wrong.

But eating potato cannot be wrong! Why? Here is the generalization:

  1. For any x, if everybody eats x in one day, then the world would be out of x.

  2. If the world is out of x, then the action "eating x" will not be possible.

  3. Then for any x, eating x cannot be universalized.

  4. Then for any x, eating x is wrong.

  5. Then eating is wrong.

  6. If everybody does not eat, then everybody will die.

  7. Then the action "not eating" will not be possible.

  8. Then "not eating" cannot be universalized.

  9. Then "not eating" is wrong.

  10. Then "eating" and "not eating" are wrong at the same time.

  11. Then universalizability principle is wrong.

There may be a problem in the order of for all x for all y where x is food and y is person, however I think this is not my mistake, but Kant's. If we switch those places, then we have something like: "all humans should not eat the same thing at the same time", which is very different than universalizability principle as the statement do not consider the human equality. However, this does not create a contradiction.

Am I wrong somewhere or did I just prove that Kant's universalizability principle is wrong because "if everyone had the same duty, then the world would be contradictory, but if everyone had different duties, then there is no contradiction"?

  • I think Kant is about the hypothetical - not the practical reasons such that you have pointed out. Sep 14, 2014 at 12:26
  • 1
    Out of the specific comes the general, so you merely have not gotten to the supposition that forms the moral clause of the argument: it would be wrong to act in such a manner as to willfully cause a famine.
    – rohme
    Aug 7, 2017 at 21:35
  • i do not think you are interpreting Kant charitably, that hardly sounds like a critique than a trivia. Oct 28, 2020 at 22:39
  • This goes to show that when Kant asks the question, "What if everyone did that?" about a proposed action, he's not endorsing that question as THE essence of HIS universality test, but showing how his is grounded, albeit deficiently, in the other. HIS test is much more Newton-fanboy mode, in that judging an action to be consistent with a moral universal law of nature involves judging the action's place in the whole slew of the categories, as the categories are the form of the laws of nature (hence "categories of freedom," here...). Oct 29, 2020 at 19:08

4 Answers 4


the categorical imperative asks whether the maxim of your action could become one that everyone could act upon in similar circumstances. If the action could be universalized (i.e., everyone could do it), then it is morally acceptable

Kant's principle only applies to the maxim of your action. Eating a potato in and of itself is not a maxim nor does it involve any kind of moral action. But yes, if Kant's principle did not state this requirement, there would be numerous ways using non-maxims to "foil" Kant's principle.

  • A maxim is a principle that one gives to oneself when acting. It states what one is going to do and why.
  • A maxim is universalizable if one's goal could be achieved in a world in which everyone acted on the maxim.

(borrowed from [1])

Establish what your maxim is, and then re-investigate your argument. I'm having difficulty conceiving of an example of a potato-based maxim that would violate Kant's principle.

EDIT in response to comment:

I think you are mixing up the theoretical reasoning used to determine whether an action moral, and the outcomes should XYZ actions actually occur. In other words, Kant's principle determines moral correctness through theoretical situations; the resulting action taken is outside the scope of his principle. For example, the classic example Kant uses in Groundwork is of a person who seeks to borrow money without intending to pay it back, whereupon he contends that a maxim of this action could not possibly be held as a universal law of nature because then no one would ever lend money anymore. This scenario he raises and his reasoning behind it remains valid whether or not there are actually other people alive to borrow money from. You may be the last person alive in the world, or just currently hiking alone in the desert, or even just mowing the lawn by yourself with no one around to lend you money — but just because you cannot currently find a person to borrow money from or find a potato to eat doesn't mean Kant's principle is flawed. It asserts action only in situations where applicable; that is, if you are in a situation where you can borrow money, or if you are in a situation where you can eat a potato, then you ought (or ought not) to, etc. If the last potato is eaten, you will no longer be in the situation the maxim describes and thus the principle no longer applies to you.

  • 1
    Thanks for the answer. My maxim is "I may eat potato to alleviate my hunger" or in a more general form "I may eat x to alleviate my hunger" where x is any kind of food. And my point is, if everyone may eat potato, then there's no problem; however if everyone eats potato, then the action cannot be generalized. Nov 14, 2013 at 7:41
  • Updated my post. Let me know if that makes any sense.
    – stoicfury
    Nov 14, 2013 at 12:36
  • @ZaferSernikli: Then maybe that maxim is indeed bad. Rather, make the maxim: "As long as there are enough potatoes that eating one would not prevent growing new ones, I can eat one to alleviate my hunger." This changed maxim still allows you to eat a potato in the real world (because there's no danger that your eating endangers the production of new potatoes) but as general principle would prevent your scenario (as people would stop eating potatoes as soon as they start getting endangered). And I think it would indeed be a morally superior maxim.
    – celtschk
    Mar 16, 2018 at 8:49
  • Late commentary, but it may be worth mentioning that, for Kantian thought, if one ought to do a thing, this implies that it has to be possible to do that thing ("ought implies can"), so any examples leading to "impossible duties" are moot, since "impossible duty" is oxymoronic. Thus, if a proper maxim (see this answer of mine) leads to something that is impossible to fulfil, this indeed does imply that the action is not moral.
    – Philip Klöcking
    Oct 29, 2020 at 8:19

The issue that I think you have had is that Kant doesn't claim that the action should be universalised right away. i.e. if you eat a potato this doesn't mean everyone has to eat them that day and the world will run out of potato...it just means there is nothing wrong with eating a potato and that the world would be fine if everyone were to eat potato..whenever they wish. As well as this, Kant's argument is concerned with questions of a moral basis. His theory does not expect every action such as 'getting into bed' or 'having a cup of tea' to be universalised. However if you are suggesting that it were just by chance that we ran out of a particular commodity then there would no longer be any need to apply Kant's principle to it, as there would be no commodity by which to apply the principle to in the first place. As well as this, when you say that 'eating x' is wrong and therefore 'eating' is wrong, this is illogical. Eating anything other than x can still be universalised.


I support your idea!

Instead of potatoes let's use an animal like elephants. If everyone kills elephants, basically for their tusks, then the world will run out of elephants. If the world runs out of elephants then we just lost a species of animal in the world.

Therefore, killing elephants is wrong!

Now let's relate this case to that of cattle. If everyone kills cattle for beef, then the world will run out of cattle and there would be no beef. Therefore killing any cattle is wrong.

  • I made an edit which you may roll back or continue editing. You can see the versions by clicking on the "edited" link above. Sep 1, 2018 at 0:55

We need to take into consideration the ways that a maxim can fail the categorical imperative "test." There are, basically, two different ways:

  1. A maxim, when universalized as a law of nature, could result in a logically impossible world. In this case, the maxim cannot logically be carried out and so is self-undermining (resulting in a perfect duty).

  2. A maxim, when universalized, could result in a world that is irrational to choose (resulting in an imperfect duty).

Consider, as a contrast case, Kant's famous perfect duty to refrain from false promises. Kant's reasoning goes like this. If everyone makes false promises when it will get them out of a bind, then the practice of promising would be destroyed. For within the very concept of a promise, the concept necessarily entails a default trust; but such a trust does not exist in a world in which everyone deterministically makes false promises. So the maxim would be a priori impossible to carry out in such a world. So the universalized maxim cannot logically carry itself out, since such a world is conceptually impossible.

Now, it seems to me that you are proposing that the maxim, "When I'm hungry, I eat potatoes" fails at test (1). But it doesn't. There is no conceptual impossibility here. Such a maxim only results in a practically impossible world, not a logically impossible one. It is perfectly conceivable that there could be a world in which there are enough potatoes for everyone in the world to eat one a day. And in fact it may be that technological advances in farming indeed make such a world the case. There may be practical limitations, but the maxim doesn't logically undermine itself. There is no logical contradiction in a will that wills potato eating universally.

Does the potato maxim fail at test (2)? It's not at all clear that it would be irrational to choose to live in a world in which everyone eats potatoes when they're hungry. But suppose we think that such a world would be irrational to choose, say, because it puts an undue burden on potato farmers. Then we have an imperfect duty not to eat potatoes everyday. But an imperfect duty is one that gives us a great deal of flexibility in how we choose to carry it out.

So at most, we have an imperfect duty not to eat a potato whenever we're hungry. But that leaves us a lot of room to eat potatoes and it's up to the individual to decide how much effort he or she puts into that obligation.

In short, you have misconceived what it takes to fail the categorical imperative test. Your premise, 'For any x, if everybody eats x in one day, then the world would be out of x' is not a conceptual truth and so it requires empirical reasoning. But it's a matter of a priori impossibility, not empirical impossibility, when considering the categorical imperative.

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