2.NounThe property of a logical theory that whenever a wff is valid then it must also be a theorem. Symbolically, letting T represent a theory within logic L, this can be represented as the property that whenever
T
⊨
ϕ
{\displaystyle T\vDash \phi }
is true, then
T
⊢
ϕ
{\displaystyle T\vdash \phi }
must also be true, for any wff φ of logic L.