A posts cript on fallacies

Bolzano is justly esteemed for his opposition to psychologism in logic. It is most fitting, therefore, that his defmition of consequence has enabled us to strike a blow at the residual psychologism that is found in the customary treatment on enthymemes.’ We shall now do the same for the so-called formal fallacies. Our discussion is a response to Gerald Massey’s claim that “fallacies . . . are perhaps of more interest to psychologists and psychiatrists than to logicians and philosophers.“* The outline of Massey’s argument is as follows: He holds that the “cardinal principle that undergirds the application of formal logic to natural languages” is that “arguments that instantiate valid argument forms are valid.“’ The naive account of formal fallacy supposes that demonstrations of invalidity proceed analogously, i.e., by showing that the argument in question instantiates an invalid form, a mistake “so common as virtually to escape notice.“4 This does not work, for, merely to point out that an argument is an instance, for example, of “asserting the consequent” will not establish its invalidity, since some arguments of that form are valid, e.g., ‘A 3 A, A != A’. To show invalidity, Massey holds, one needs to show not just that an argument is an instance of an invalid form, but that it fails to instance a valid form, a vastly more difficult undertaking which flounders on the “apparently unfinished state of contemporary logic . . . . For even if it could somehow be established that no translation of an argument into formal systems yields a valid argument form, how could one hope to prove the same result about all possible systems?“5 There is then no logical theory of fallacy. But, he thinks, one can still speak of fallacies in a different, namely a psychological, sense. Consider the following example: