Take ordinary material logical implication on , designate , and interpret negation by the constant-zero operation . Then and are always designated, and modus ponens rule and logical implication discharge preserve consequence. But is false for . Thus the first two schemes and the deduction rule do not derive the last scheme. This detects the loss of a genuine classical-negation principle even though all the displayed schemes are tautologies under the usual classical negation.
Articles by others on the same topic
There are currently no matching articles.