Logic in practice / by L. Susan Stebbing.

  • Stebbing, L. Susan (Lizzie Susan), 1885-1943.
Date:
1934
Catalogue details

Licence: In copyright

Credit: Logic in practice / by L. Susan Stebbing. Source: Wellcome Collection.

    44/140 (page 30)
    A : ‘I don't see any difference between All blasting sounds like that and All that sounds like that is blasting, except that the second way of putting it is very clumsy.’ B : ‘ Do you see any difference between All seals are mammals ’ and ‘ Only seals are mammals ? ’ A : ‘Of course. The first is true, and the second isn’t.’ B : ‘ Why do you say the second proposition isn’t true ? ’ A : ‘ Because men, and horses, and elephants, and a lot of other animals are mammals as well as seals.’ B : ‘ Then it doesn’t follow from All seals are mammals that All mammals are seals ? ’ 1 A : ‘Of course not.’ B : ‘ Then you ought to admit that it doesn’t follow from the fact that all blasting operations make a certain sort of sound, that whatever makes that certain sort of sound is a blasting operation.’ It is to be hoped that B’s argument may have convinced the reader, whatever may have been the case with A. The point that concerns us is why B began to talk about seals and mammals in order to show A that his reason for holding that a certain sound was due to blasting was not a good reason. There is no connexion between blasting and sounds, on the one hand, and seals and mammals, on the other. B's purpose was to call A’s attention to the form of what he said, since, if the reason he offered was a good reason, it must be a good reason in any other argument of the same form. Now all seals are mammals 1 Here B assumes that if what follows from a given proposition is false, then that given proposition is also false, and A’s next remark accepts this assumption.