Other Minds, Reason, Morality

Some philosophers have been challenged by the problem of the existence of other minds, primarily because immediate evidence of such an existence seems lacking.  Now, according to Kant, entailed by his principle of Pure Practical Reason is the principle to treat an other, because rational, as an end-in-itself.  In other words, he implicitly classifies the question of the existence of other minds as a Moral, not a Cognitive, problem, and solves it accordingly.