First-order Logic
A set of formulas in first-order logic is consistent (written Con) if and only if there is no formula such that and . Otherwise is inconsistent and is written Inc.
is said to be simply consistent if and only if for no formula of, both and the negation of are theorems of .
is said to be absolutely consistent or Post consistent if and only if at least one formula of is not a theorem of .
is said to be maximally consistent if and only if for every formula, if Con then .
is said to contain witnesses if and only if for every formula of the form there exists a term such that . See First-order logic.
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“Logic is not a body of doctrine, but a mirror-image of the world. Logic is transcendental.”
—Ludwig Wittgenstein (1889–1951)