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Full Idea
'Γ |=' means 'Γ is a set of closed formulae, and there is no (standard) interpretation in which all of the formulae in Γ are true'. We abbreviate this last to 'Γ is inconsistent'.
Gist of Idea
A set of formulae is 'inconsistent' when there is no interpretation which can make them all true
Source
David Bostock (Intermediate Logic [1997], 4.5)
Book Ref
Bostock,David: 'Intermediate Logic' [OUP 1997], p.165
A Reaction
This is a semantic approach to inconsistency, in terms of truth, as opposed to saying that we cannot prove both p and ¬p. I take this to be closer to the true concept, since you need never have heard of 'proof' to understand 'inconsistent'.
10071 | Second Incompleteness: nice theories can't prove their own consistency [Gödel, by Smith,P] |
13341 | Using the definition of truth, we can prove theories consistent within sound logics [Tarski] |
13540 | A set of formulae is 'inconsistent' when there is no interpretation which can make them all true [Bostock] |
13541 | For 'negation-consistent', there is never |-(S)φ and |-(S)¬φ [Bostock] |
13542 | A proof-system is 'absolutely consistent' iff we don't have |-(S)φ for every formula [Bostock] |
12656 | P-and-Q gets its truth from the truth of P and truth of Q, but consistency isn't like that [Fodor] |
9552 | Sentences are consistent if they can all be true; for Frege it is that no contradiction can be deduced [Chihara] |
18785 | Consistency is semantic, but non-contradiction is syntactic [Mares] |
10119 | Consistency is a purely syntactic property, unlike the semantic property of soundness [George/Velleman] |
10126 | A 'consistent' theory cannot contain both a sentence and its negation [George/Velleman] |