A Kantian Deduction
Suppose Bill and Megan invite their friend Doris to go to the movies with them on Friday. Unfortunately, Doris does not have enough money to pay for a movie. Bill and Megan offer to pay her way provided that she pay them back on Monday. Doris--poor student that she is--knows that she will be unable to pay them back next week, much less Monday. She wonders to herself, 'should I lie and tell them that I can pay them back on Monday?' Here is how Kant answers the question.
| 1 | I should lie. | Premise (Doris's maxim) | |
| ∴ | 2 | Everyone should lie. | Categorical Imperative, 1 |
| ∴ | 3 | My friends should lie to me. | 2 |
| 4 | If I should lie then I can go to the movies. | Premise | |
| 5 | If my friends should lie to me then I cannot go to the movies. | Premise | |
| ∴ | 6 | I can go to the movies. | 1&4 |
| ∴ | 7 | I cannot go to the movies. | 3&5 |
| ∴ | 8 | I can go to the movies and I cannot go to the moves. | 6&7 |
| ∴ | 9 | I should not lie. | 1&8 |
Notice that the argument is valid: it is in the Reductio ad Absurdum form. Notice also that the conclusion, "I should not lie", is categorical. In other words, it is not a hypothetical imperative. If not lying is equivalent to telling the truth, then one more application of the Categorical Imperative--which, for the purposes of a Kantian Deduction, we should think of as a rule of logical inference--results in universal conclusion that everyone should tell the truth. This is an absolute duty, and Kant's clever use of the Reductio ad Absurdum form in conjunction with the first formulation of the Categorical Imperative is how he arrives at the implications of his theory.