We will now enumerate a few duties, adopting the usual division of them into duties to ourselves and to others, and into perfect and imperfect duties. [Footnote:  It must be noted here that I reserve the division of duties for a future metaphysic of morals; so that I give it here only as an arbitrary one (in order to arrange my examples).  For the rest, I understand by a perfect duty one that admits no exception in favour of inclination, and then I have not merely external, but also internal perfect duties.  This is contrary to the use of the word adopted in the schools; but I do not intend to justify it here, as it is all one for my purpose whether it is admitted or not. [Perfect duties are usually understood to be those which can be enforced by external law; imperfect, those which cannot be enforced.  They are also called respectively determinate and indeterminate, officia juris and officia virtutis.]]

I. A man reduced to despair by a series of misfortunes feels wearied of life, but is still so far in possession of his reason that he can ask himself whether it would not be contrary to his duty to himself to take his own life.  Now he inquires whether the maxim of his action could become a universal law of nature.  His maxim is:  From self-love I adopt it as a principle to shorten my life when its longer duration is likely to bring more evil than satisfaction.  It is asked then simply whether this principle founded on self-love can become a universal law of nature.  Now we see at once that a system of nature of which it should be a law to destroy life by means of the very feeling whose special nature it is to impel to the improvement of life would contradict itself, and therefore could not exist as a system of nature; hence that maxim cannot possibly exist as a universal law of nature, and consequently would be wholly inconsistent with the supreme principle of all duty. [Footnote:  On suicide cf. further Metaphysik der Sitten, p. 274.]

2.  Another finds himself forced by necessity to borrow money.  He knows that he will not be able to repay it, but sees also that nothing will be lent to him, unless he promises stoutly to repay it in a definite time.  He desires to make this promise, but he has still so much conscience as to ask himself:  Is it not unlawful and inconsistent with duty to get out of a difficulty in this way?  Suppose, however, that he resolves to do so, then the maxim of his action would be expressed thus:  When I think myself in want of money, I will borrow money and promise to repay it, although I know that I never can do so.  Now this principle of self-love or of one’s own advantage may perhaps be consistent with my whole future welfare; but the question now is, Is it right?  I change then the suggestion of self-love into a universal law, and state the question thus:  How would it be if my maxim were a universal law?  Then I see at once that it could never hold as a universal law of nature, but would necessarily contradict itself.  For supposing it to be a universal law that everyone when he thinks himself in a difficulty should be able to promise whatever he pleases, with the purpose of not keeping his promise, the promise itself would become impossible, as well as the end that one might have in view in it, since no one would consider that anything was promised to him, but would ridicule all such statements as vain pretences.