 Deduction Theorem  If \(B\) is derivable from \(A\) and possibly other formulas \(F,\) with all variables free in \(A\) held constant in the derivation (that is, without using the second or third rule of inference on any variable \(x\) occurring free in \(A,\) unless the assumption \(A\) does not occur in the derivation before the inference in question), then \(A \rightarrow B\) is derivable from \(F.\) 