Intensional Fallacy

In philosophical logic, the intensional fallacy is committed when one makes an illicit use of Leibniz's law in an argument. Leibniz's law states that, if one object has a certain property, while another object does not have the same property, the two objects cannot be identical. For example, if Clark Kent can fly, and Lois Lane cannot fly, they must be different people. The intensional fallacy occurs when this argument is used with properties that are intensional, that is, when the property depends on the description of the object used. In this case, although the object may have a given property under one description, and not have that property under a different description, they may nonetheless be the same object. An example of the intensional fallacy would be:

• Lois Lane believes that Superman can fly.
• Lois Lane does not believe that Clark Kent can fly.
• Therefore Superman and Clark Kent are not the same person.

The conclusion in this case is false, because what Lois Lane believes depends not on the actual object (Clark Kent/Superman), but rather on the name used.

Intensional sentences are extensionally opaque. These sentences are often intentional (with a 't'), that is they involve a property of the mind that is directed at an object. I can coherently hold at the same time: "I believe that Muhammed Ali was the heavyweight champion" and "I don't believe that Cassius Clay was heavyweight champion" if I don't know that Muhammed Ali and Cassius Clay were the same person. In my belief, Muhhamed Ali and Cassius Clay fail in their extension.

This fallacy was cited by authors Jerry Fodor and Massimo Piattelli-Palmarini in their book What Darwin Got Wrong.

This fallacy is also known as the "epistemic fallacy." See Critical Thinking: A Concise Guide (3ed) by Tracy Bowell and Gary Kemp. p. 225.