Reasoning by Analogy.
Reasoning by analogy is an elementary form of reasoning, and is the particular kind of reasoning employed by the majority of persons in ordinary thought. It is based upon the unconscious recognition by the human mind of the principle which is expressed by Jevons as: "If two or more things resemble each other in many points, they will probably resemble each other in more points." The same authority says: "Reasoning by analogy differs only in degree from that kind of reasoning called 'generalization.' When many things resemble each other in a few properties, we argue about them by generalization. When a few things resemble each other in many properties, it is a case of analogy."
While this form of reason is frequently employed with more or less satisfactory results, it is always open to a large percentage of error. Thus, persons have been poisoned by toadstools by reason of false analogous reasoning that because mushrooms are edible, then toadstools, which resemble them, must also be fit for food; or, in the same way, because certain berries resemble other edible berries they must likewise be good food. As Brooks says: "To infer that because John Smith has a red nose and is also a drunkard, then Henry Jones, who also has a red nose, is also a drunkard, would be dangerous inference. Conclusions of this kind drawn from analogy are frequently dangerous." Halleck says: "Many false analogies are manufactured, and it is excellent thought training to expose them. The majority of people think so little that they swallow these false analogies just as newly-fledged robins swallow small stones dropped into their mouths."
Jevons, one of the best authorities on the subject, says: "There is no way in which we can really assure ourselves that we are arguing safely by analogy. The only rule that can be given is this: That the more closely two things resemble each other, the more likely it is that they are the same in other respects, especially in points closely connected with those observed. In order to be clear about our conclusions, we ought, in fact, never to rest satisfied with mere analogy, but ought to try to discover the general laws governing the case. * * * We find that reasoning by analogy is not to be depended upon, unless we make such an inquiry into the causes and laws of the things in question that we really employ inductive and deductive reasoning."