This is "a logical question that forces us to confront whether the act would lead to a self-contradiction." Kant says that we simply can 't excuse our own behavior when it contradicts the universal. Because the universal ethical idea is true under any circumstance it is considered categorical. It is a command or an imperative once we realize that the particular ethical issue is true. We must do it.
To follow Kant 's philosophy at all times, in all situations, is not actually something that works universally. If lying is always wrong in all situations, then when the Nazi 's came to the house of a German family that was hiding Jews and they asked if they had any Jews inside, following Kant 's philosophy, they would have to answer, yes. Because to lie would go against the categorical imperative. In this view there is no exceptions to the rule. The idea that the saving of life doesn 't supersede the law of not