Full Idea
The law of excluded middle is the reflection, within logic, of the principle of bivalence. It states that 'For any statement A, the statement 'A or not-A' is true'.
Clarification
Bivalence says sentences can only be 'true' or 'false'
Gist of Idea
The law of excluded middle is the logical reflection of the principle of bivalence
Source
Michael Dummett (Thought and Reality [1997], 5)
Book Reference
Dummett,Michael: 'Thought and Reality (Gifford Lectures)' [OUP 2006], p.62
A Reaction
True-or-not-true is an easier condition to fulfil than true-or-false. The second says that 'false' is the only alternative, but the first allows other alternatives to 'true' (such as 'undecidable'). It is hard to challenge excluded middle. Somewhat true?