Probably no name has more weight in the history of European philosophy than Kant’s. With this time-honored name, the concept of the categorical imperative immediately dawns: “Only act according to the maxim through which one can also want it to become a general law”. But how can you imagine putting this into practice?
Brief repetition – categorical imperative
In the first part of this article we sketched the main features of Kant’s deontological ethics and stopped examining maxims, the subjective principles of action. I want to briefly explain the basis for this again at the beginning of the second part, because the following examples for checking morality will build on it.
Kant’s ethics revolves around what is essential to practical reason anyway, namely the demand for maxims to be universalized. According to Kant, our ability to will is determined by the fact that we derive subjective maxims from objective principles. 
There is immoral action precisely when my maxim can only apply as an exception. On the other hand, moral action is something that goes hand in hand with the categorical imperative: only act according to the maxim by which you can also want it to become a general law . 
For us, this expression “can will” becomes important – what does Kant mean by that?
Two types of contradictions
You have to understand it this way: the categorical imperative is a test procedure for our maxims. In this way we can never find positive determinations of moral action, but we can always sort out maxims that do not meet the criterion of universalizability.
The ability to want is the touchstone for this. Whether a maxim can be a general law is determined by the occurrence of two kinds of contradictions, which can become obvious when generalizing:
(1) A contradiction in thinking: A maxim is forbidden if, as a law in a world, it leads to a logical contradiction in itself.
(2) A contradiction in willingness: a maxim is forbidden if, as a law in a world, it leads to the will of this maxim preventing itself.
It is best to demonstrate these two contradictions using examples. In his work Fundamentals of the Metaphysics of Morals, Kant agrees to four cases – that is, four maxims – which he examines according to the categorical imperative. In this way he tries to determine the general moral duties:
Let us now count some duties according to the usual division of these into duties towards ourselves and towards other people, into perfect and imperfect duties. [2, 421]
Suicide – Case 1:
The first case that Kant cites to illustrate the categorical imperative is this [2, 422]:
Someone who has grown tired of life through a series of evils that has grown to the point of hopelessness is still sufficiently in control of his reason that he can ask himself whether it is not against his duty to himself either to take your own life.
Now he tries: whether the maxim of his action could well become a general law of nature. But his maxim is: Out of self-love, I make it my principle when life, with its long term, threatens more evil than it promises to be pleasant, to shorten it for me.
The question arises whether this principle of self-love could become a general law of nature.
For Kant, the generalization of this maxim results in a contradiction in thinking, i.e. a clear logical inconsistency. A nature that would act according to such a law would inevitably have to destroy itself and could therefore not exist in advance. So the maxim cancels itself:
But then one soon sees that a nature, whose law it would be, by the same sensation whose purpose it is to drive life to destroy life itself, contradict itself and therefore not exist as nature, hence that maxim could not possibly take place as a general law of nature, and consequently completely contradict the highest principle of all duty.
Opportunistic Liar – Case 2:
An example that has more to do with our everyday life is given by Kant as the second case [2, 422]:
Another feels forced to borrow money through hardship. He knows well that he will not be able to pay, but also sees that nothing will be lent to him now if he does not make a firm promise to pay for it at a certain time. He feels like making such a promise.
But does he have enough conscience to ask himself: is it not forbidden and contrary to duty to help himself out of trouble in this way? Assuming he decided to do so, his maxim for the action would be this: if I think I am in need of money, I will borrow money and promise to pay for it, although I know right away that it will never happen.
Here too, according to Kant, there is again a contradiction in thinking. A world that had it as a general law to always lie with faked promises in principle would now abolish the concept of the promise as such. There would no longer be such a thing as a promise, and consequently one could no longer lie to anyone. So the maxim contradicts itself:
Because the generality of a law that everyone, after he believes he is in need, can promise whatever he can think of, with the resolution not to keep it, would make the promise and the purpose one might have with it impossible, because nobody would believe that something was promised to him, but would laugh at all such statements as vain pretense.
The idler – case 3:
I find the last two cases which Kant cites to test maxims more interesting [2, 423]:
A third person finds in himself a talent which, through some culture, could turn him into a man useful for all kinds of purposes. But he sees himself in comfortable circumstances, and prefers to indulge in pleasure rather than endeavor to expand and improve his happy natural dispositions. He still asks, however, whether, besides the correspondence which his maxim of neglecting his natural gifts has in itself with his penchant for pleasure, it also agrees with what is called duty.
Problematisation of the third case
There is obviously no contradiction in thinking in this case. A world in which everyone would wither their talents could certainly exist (even if it weren’t very beautiful). In any case, there is no contradiction between the maxim and itself. But there is another problem:
But he cannot possibly want this to become a general law of nature, or to be laid in us as such by natural instinct. For as a rational being he necessarily wants all faculties to be developed in him because they are useful and given to him for all kinds of possible purposes.
According to Kant, it is impossible to want this maxim to become universal. It is important to understand that being able to will does not ask whether we would like everyone to act like this or whether the world is a good place. All this depends on inclination, which has no place in a rational justification of morality.
By being able to want, Kant means whether it is possible without contradiction. And that does not seem to be the case in this example. After all, even a world of lazy people has to pursue certain goals which, however, would become a hindrance if talents were neglected. It is therefore impossible to want this maxim to be universal.
The Inhuman Egoist – Case 4:
Finally, Kant gives a case analogous to the solution [2, 423]:
A fourth, who is well, still thinks, while he sees that others are struggling with great toil (which he could probably help too): what is it to me? everybody may be as happy as heaven wants, or he can make himself, I will not withdraw anything from him, not even envy him; only for his well-being, or his support in need, I don’t feel like contributing anything!
Here, too, we could imagine a world that, according to this same maxim, would endure without contradictions. It would be a pretty terrible world, but as I said before, our personal opinion about it doesn’t matter. Kant also emphasizes this again:
Now, of course, if such a way of thinking became a general law of nature, the human race could very well exist, and no doubt even better than if everyone chatted about sympathy and benevolence, also eager to practice such things occasionally, but also wherever they were can, cheat, sell people’s rights or otherwise harm it.
I can’t want to refuse to help
Analogous to the third case, there is also a contradiction in willingness. It is simply undesirable that we should exercise a universal principle of egoism, because cases will certainly occur where we are dependent on the help of others, even if it is only about achieving our own goals. According to this, such a maxim of refusing help and assistance in principle would be impossible:
But, although it is possible that according to that maxim a general law of nature could well exist, it is still impossible to want such a principle to apply everywhere as a law of nature. Because a will that resolves this would conflict with itself, in that some of the cases can discern where it needs other love and participation, and where, through such a law of nature arising from its own will, it gives itself all the hope of assistance that he wishes he would steal.