I am no Kant scholar (thankfully!) but I do regularly teach ethics and so am constantly re-reading the Groundwork of the Metaphysics of Morals. I have never really known what to think about Kant’s argument in the section where he is discussing the formula of the law of nature and how it applies to suicide. Here is what he says:
…[the] maxim…is: from Self-love I make it my principle of action to shorten my life when its longer duration threatens more troubles than it promises agreeableness. The only further question is whether this principle of self-love could become a universal law of nature. It is then seen at once that a nature whose law it would be to destroy life itself by means of the same feeling whose destination is to impel toward the furtherance of life would contradict itself and would not therefore subsist as nature…
I have always interpreted this passage as expressing some kind of natural law theory rather than a deliverance of the categorical imperative. The function or role of self-love is the preservation of life. Using it in this way violates that natural function and so results in a contradiction of a sort but not the usual sort you find in the case of perfect duties. Of course the flaw in that reasoning is that it is not obvious why it must be the case that to use something contrary to its function is immoral (is it immoral to use a butter knife to screw in a screw?).
For full disclosure, I take myself to be a Kantian of a sort, though I would probably not be viewed as an orthodox Kantian (I, for instance, think we can derive Utilitarianism from the Categorical Imperative (like Hare did as well) and I think we can derive duties towards animals in much the same way). And I also take it that suicide is morally permissible in at least some cases and so I have always been happy to see something odd about Kant’s argument against suicide. Yet today in class I found myself wondering whether this natural-law-remnant theory is the most charitable reading of that passage.
In the case of the other perfect duties we seem to find a pattern where in the imagined world the very thing that we are trying to do becomes impossible. Thus in the lying promise case the very act of making a promise becomes impossible. Since we cannot be coherently described as promising in a world where everyone makes lying promises we arrive at a contradiction. So, too, in the case of lying in general. In a world where everyone lies to avoid discomfort lying itself is impossible. So too for stealing. In a world where everyone steals stealing is itself impossible (since there is no ownership in such worlds and when we steal we take ourselves to be owners). So if we are to extend this general line to what Kant says here then it must somehow be the case that in the world where the maxim in question is a universal law suicide itself is impossible.
This seems implausible but it might not be. Suicide is the act of taking one’s life out of self-love. If self-love is itself not possible then suicide is not possible. So maybe what Kant had in mind was that self-love is by definition the thing which impels us towards life and so in the suicide-maxim world there is no self-love since there is no universal drive towards life, but rather only a limited drive towards life-as-long-as-it-is-agreeable or some such. Since there is no self-love there can be no suicide (which requires that you act from self-love). This would still be a questionable argument (since the limited thing still seems like self-love enough to do its job in ‘normal’ circumstances) but might be a more fair way of reading Kant’s claim in the above passage.