The law-courts are often rightly blamed for being strenuous opponents of the spread of modern logic: the frequent misuse of and, or, the, and provided that in them is notorious. But the fault seems partly to lie in the uncomplicated nature of the logical problems which are dealt with in them. Thus it is no uncommon thing for somebody to appear there who is unable to establish his own identity, or for A to assert that B was “not himself” when he made a will leaving his money to C.
The chief use of identities is in implication. Since, in logic, we so understand implication[21] that any true proposition implies and is implied by any other true proposition; if one is convinced of the truth of the proposition Q, it is advisable to choose one or more identities P, whose truth is undoubted, and say that P implies Q. Thus, Mr. Austen Chamberlain, according to The Times of March 27, 1909, professed to deduce the conclusion that it is not right that women should have votes from the premisses that “man is man” and “woman is woman.” This method requires that one should have made up one’s mind about the conclusion before discovering the premisses—by what, no doubt, Jevons would call an “inverse or inductive method.” Thus the method is of use only in speeches and in giving good advice.
Mr. Austen Chamberlain afterwards rather destroyed one’s belief in the truth of his premisses by putting limits to the validity of the principle of identity. In the course of the Debate on the Budget of 1909, he maintained, against Mr. Lloyd George, that a joke was a joke except when it was an untruth: Mr. Lloyd George, apparently, being of the plausible opinion that a joke is a joke under all circumstances.
[20] The argument about the hull of a ship disappearing first is not convincing, since it would equally well prove that the surface of the earth was, for example, corrugated on a large scale. If the common-sense of the reader were supposed to dismiss the possibility of water clinging to such corrugations, it might equally be supposed to dismiss the possibility of water clinging to a spherical earth. Traditional geography books, no doubt, gave rise to the opinions held by Lady Blount and the Zetetic Society.
[21] The subject of Implication will be further considered in Chapter XIX.