A new logic / by Charles Mercier.

  • Mercier, Charles Arthur, 1852-1919.
Date:
1912
Catalogue details

Licence: In copyright

Credit: A new logic / by Charles Mercier. Source: Wellcome Collection.

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    2$2 to ‘ The Marquis of Salisbury was First Lord of the Treasury,’ it is clear that this inference is not based solely upon the postulate, that the Marquis was prime minister. I must have the further postulate that the prime minister is equivalent to the First Lord of the Treasury. If every building in the town was destroyed by the earthquake, I may infer from this postulate that the town hall was destroyed ; but I cannot do so until I have assumed the further postulate that the town hall was a building in the town. If from the postulate that it was cheap at fifty guineas I infer that it was cheap at fifty pounds, I make the silent assumption that fifty pounds is less than fifty guineas. It is true that in each case the silent premiss is so universally known, is so intimate a part of our mental equipment, that we do not recognise that we are making any assumption at all; but it is there, nevertheless; and strictly speaking, all these inferences are drawn, not from simple, but from compound propositions. But so to consider them would be pedantic formalism. For all practical purposes, they are inferences from a single postulate; or, to put it otherwise, and more accurately, we are entitled, when a postulate is granted to us, to add to it what knowledge of its terms is already in our possession. Otherwise, the replaceability or non-replaceability of one term by another depends on the purpose of the argument; and the purpose of the argument is extra-logical, and is determined by other considerations. Is china clay a mineral ? That depends on the purpose of the argument. For the purpose of Geology, it is ; but for the purpose of a lease of minerals, the Courts of Law must be invoked to determine the question. Is marriage with a sister-in-law a valid marriage, or not ? For the purpose of civil law it is. For the purpose of the forum conscitntice it may or may not be, according to the conscience of the person con- cerned. For the purpose of administration of the Sacrament, it is a valid marriage in law; it is not a valid marriage in the opinion of some of the clergy. Is the bursting of an aneurism equivalent to an accident ? For the purpose of medicine, it is not. For the purpose of the Workmen’s Compensation Act, it is ; for the purpose of an accident insurance policy, it may or may not be. These are questions that Logic cannot answer on the basis of a single postulate. They depend on the purpose of the argument, and for every different purpose, different accessory postulates are