Full Idea
The maxim of freely disposing of my life could not hold as a universal law of nature, …because no one could choose to end his life, for such an arrangement could not constitute a permanent natural order.
Gist of Idea
A permanent natural order could not universalise a rule permitting suicide
Source
Immanuel Kant (Critique of Practical Reason [1788], I.1.1.I)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.45
A Reaction
This sort of claim brings out the advantanges of Aristotelian 'particularism' (expounded by Dancy). Obviously universal suicide isn't promising, but no one wants that. A few suicides in extreme cases will have no effect at all on the natural order.