The Argument from Reason

1. Human cognitive faculties are generally reliable. (Premise)

2. If human cognitive faculties are caused by rational processes, then they may be generally reliable. (Premise)

3. If human cognitive faculties are caused by non-rational processes, then the cognitive faculties' general reliability is inscrutable. (Premise)

4. The reliability of human cognitive faculties is scrutable. (Premise)

5. Therefore, it is most rational to believe that human cognitive faculties are caused by rational processes. (From 1 - 4)

Now, what if human cognitive faculties are not generally reliable?

6. Human cognitive faculties are not generally reliable. (Assumption)

7. The reliability of human cognitive faculties is less than . 5. (Implied by 1)

8. (6) is a belief formed by unreliable cognitive faculties whose likelihood of truth is less than .5. (From 6 and 7)

9. Hence, it is irrational to believe that human cognitive faculties are generally unreliable. (From 6 and 8)

10. Therefore, it is most rational to believe that human cognitive faculties are generally reliable. (Implied by 9)

Of course, these "rational processes" could be explained away by aliens.  However, the theist could easily supplement the argument from reason with the argument from motion for the existence of an Unmoved Mover.  The same logic would follow, and we may sit comfortably knowing that we have a sound argument for an intelligent Unmoved Mover.