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The use of this fact forms the basis of a proof technique called proof by contradiction, which mathematicians use extensively to establish the validity of a wide range of theorems. This applies only in a logic where the law of excluded middle A ∨ ¬ A {\displaystyle A\vee \neg A} is accepted as an axiom.
Formally the law of non-contradiction is written as ¬(P ∧ ¬P) and read as "it is not the case that a proposition is both true and false". The law of non-contradiction neither follows nor is implied by the principle of Proof by contradiction. The laws of excluded middle and non-contradiction together mean that exactly one of P and ¬P is true.
A sentence can be viewed as expressing a proposition, something that must be true or false. The restriction of having no free variables is needed to make sure that sentences can have concrete, fixed truth values : as the free variables of a (general) formula can range over several values, the truth value of such a formula may vary.
Consistent sentence: A sentence of is consistent if it is true under at least one interpretation. It is inconsistent if it is not consistent. [ 66 ] [ 68 ] An inconsistent formula is also called self-contradictory , [ 1 ] and said to be a self-contradiction , [ 1 ] or simply a contradiction , [ 81 ] [ 82 ] [ 83 ] although this latter name is ...
a meaningful declarative sentence that is true or false, [citation needed] or; a proposition. Which is the assertion that is made by (i.e., the meaning of) a true or false declarative sentence. [1] [2] In the latter case, a (declarative) sentence is just one way of expressing an underlying statement.
The predicate calculus goes a step further than the propositional calculus to an "analysis of the inner structure of propositions" [4] It breaks a simple sentence down into two parts (i) its subject (the object (singular or plural) of discourse) and (ii) a predicate (a verb or possibly verb-clause that asserts a quality or attribute of the object(s)).
The same type of relationship is shown in (2), where the first sentence can be interpreted as implying that by giving a party for the new students, the hosts will serve drinks. This is, of course, a defeasible inference based on world knowledge, that is then contradicted in the following sentence.
A Gödel sentence G for a system F makes a similar assertion to the liar sentence, but with truth replaced by provability: G says "G is not provable in the system F." The analysis of the truth and provability of G is a formalized version of the analysis of the truth of the liar sentence.