Full Idea
If my maxim is 'augment my property by all safe means', I can't make that a law allowing me to keep a dead man's loan, because no one would make a loan if that were the moral law.
Gist of Idea
No one would lend money unless a universal law made it secure, even after death
Source
Immanuel Kant (Critique of Practical Reason [1788], I.1.1.§4)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.27
A Reaction
This is a simple illustration of Kant's strategy and it shows clearly how, for all his talk of 'pure reason', his moral law is strongly guided by consequences, and that these can only judged by prior values - for example, that loans are a good thing.