Logic

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. 240. True conclusions, as Aristotle has observed, can be correctly drawn from false premises. Every Laplander is a born poet, Homer was a Laplander, and therefore - by the first figure - a poet. All parasitic plants have red flowers, no rose has red flowers, therefore - by the second figure - roses are not parasitic plants. Metals do not conduct electricity, all metals are non-fusible, and hence - according to the third figure - non-fusible substances exist, which are non-conductors of electricity. Alter Laplander into Greek, plants which have red flowers into plants which have exploding seed-vessels, and write glass for metal, and in each example one premise will be true, while by inserting a new middle term in each case you may make both premises true, but in every case the conclusion follows with neither more nor less validity. Let T be a perfectly true proposition, S its subject, and P its predicate; then a middle term M may be chosen at random so long as the terms are arranged in both premises on the model of an Aristotelian figure: if this is done the conclusion T will always follow according to the figure.<br><br>We shall see why this is universally true, if we take as our middle term an abstract symbol M, instead of a concrete term: thus, all M are poets, Homer was an M; all parasitic plants are M, roses are not M; all M are non-conductors, all M are non-fusible.