law of the excluded middle

This article uses, in part, forms of logical notation. For a concise description of the notations used, see the Basic and Derived Argument Forms table here, or First-order predicate logic. In logic, the law of the excluded middle states that the formula "P ∨ ¬P" ("P or not-P") can be deduced from the calculus under investigation. It is one of the defining properties of classical systems of logic. However, some systems of logic have different but analogous laws, while others reject the law of excluded middle entirely.
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one of three principles required by logical thinking set down by Aristotle.  The law of the excluded middle tells us that all statements must be treated as either true or false.  There is no middle ground.  Click here for more information.