Intuitionism and the Law of Excluded Middle
Created | Updated Feb 27, 2006
Essay Question: Evaluate the intuitionistic/antirealist reasons for rejecting the law of excluded middle.
The law of excluded middle was held by Aristotle and later echoed by Russell as one of three "Laws of Thought":
- "The law of Identity: Whatever is, is.
- The law of contradiction: Nothing can be and not be.
- The law of excluded middle: Everything must either be or not be."
In symbolic notation, the law of exluded middle says that "(P)V(¬P)" is true.
Dummett's Anti-Transcendence and Rejection of LEM
Dummett rejects the law of excluded middle following his anti-realist argument against transcendence and his identification of truth with assertibility.
Transcendence is the view that for some statements S, it is possible that S is true but we are unable to tell that S is true. Dummett proceeds to close the gap between truth and decidability.
- Dummett's argument against Transcendence:
- Manifestation: For any S that we understand (know the meaning of), we can make manifest what we know to be its meaning.
- Meaning: The meaning of S is constituted by its truth conditions.
- :. For any S we understand, we can make manifest the truth conditions of S.
- Decidability: We can make manifest S's truth conditions only if we could tell that those conditions were fulfilled if they were.
- :. For any S we understand, if S is true we can tell that it is true. [AT or Anti-transendence]
This now requires a different view of truth, because no platonist view of truth will support anti-transendence.