Reason, Judgment, Duty

In the Groundwork, Kant distinguishes between a maxim that is inconceivable as a universal law, and a maxim the universalization of which is, while internally conceivable, is inconsistent with the internally conceivable universalization of some other maxim.  The significance of that distinction to his system is that it is the basis of the difference between the concepts of 'perfect' and 'imperfect' duty, and, subsequently, of that between those of 'Justice' and 'Virtue'.  Now, as has been previously discussed here, at #40 of the 3rd Critique, he draws a distinction between "judgment", which is thinking from a "universal standpoint", and "reason", which is "consistent" thinking.  On that basis, it is Judgment, specifically, that is the source of the universalizability of a maxim, and Reason, specifically, that is the source of the consistency between universalizations.  Accordingly, in Kant's system, it is Judgment, specifically, that is the source of the concepts of Perfect Duty and Justice, while it is Reason, specifically, that is the source of the concepts of  Imperfect Duty and Virtue.