Illicit conversion, or Affirming the consequent is the logical fallacy caused by the inversion of the subject and predicate in a proposition.
- No P are Q.
- Therefore, no Q are P.
The faulty logic which often leads to this fallacy goes something like: No dogs are cats, therefore no cats are dogs. While the conclusion is true, the premise does not necessarily lead to it. More often this will apply to something that happens to be observed now but does not apply to a condition: There is no metal in the air, therefore nothing in the air is metal. An airplane proves this conclusion false, there is nothing about being in the air that prevents an object from being metal. No cars are seven legged, therefore nothing with seven legs is a car. There is nothing about having seven legs that prevents an object from being a car.