Reasoning, the third great step in thinking, may be said to consist of ascertaining new truths from old ones, new judgments from old ones, unknown facts from known ones; in short, of proceeding logically from the known to the unknown, using the known as the foundation for the unknown which is sought to be known. Gordy gives us the following excellent definition of the term: “Reasoning is the act of going from the known to the unknown through other beliefs; of basing judgment upon judgments; reaching beliefs through beliefs.” Reasoning, then, is seen to be a process of building a structure of judgments, one resting upon the other, the topmost point being the final judgment, but the whole constituting an edifice of judgment. This may be seen more clearly when the various forms of reasoning are considered.
The simplest form of reasoning is that known as “immediate reasoning,” by which is meant reasoning by directly comparing two judgments without the intervention of the third judgment, which is found in the more formal classes of reasoning. This form of reasoning depends largely upon the application of the Three Primary Laws of Thought, to which we have referred in a previous chapter.
It will be seen that _if_ (_a_) a thing is always itself, then (_b_) all that is included in it must partake of its nature. Thus, the bird family has certain class characteristics, therefore by immediate reasoning we know that _any_ member of that family must possess those class characteristics, whatever particular characteristics it may have in addition. And we likewise know that we cannot attribute the _particular_ characteristics, as a matter of course, to the other members of the class. Thus, though all sparrows are birds, it is not true that all birds are sparrows. “All biscuits are bread; but all bread is not biscuit.”
In the same way we know that a thing cannot be bird and mammal at the same time, for the mammals form a not-bird family. And, likewise, we know that everything _must_ be either bird or not bird, but that being not bird does not mean being a mammal, for there are many other not-bird things than mammals. In this form of reasoning distinction is always made between the _universal_ or general class, which is expressed by the word _all_, and the _particular_ or individual, which is expressed by the word “some.” Many persons fail to note this difference in their reasoning, and fallaciously reason, for instance, that because _some_ swans are white, _all_ swans must be so, which is a far different thing from reasoning that if _all_ is so and so, then _some_ must be so and so. Those who are interested in this subject are referred to some elementary text-book on logic, as the detailed consideration is too technical for consideration here.
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.”
HIGHER FORMS OF REASONING.
The two higher forms of reasoning are known, respectively, as (1) inductive reasoning, or inference from particular facts to general laws; and (2) deductive reasoning, or inference from general truths to particular truths. While the class distinction is made for the purpose of clear consideration, it must not be forgotten that the two forms of reasoning are generally found in combination. Thus, in inductive reasoning many steps are taken by the aid of deductive reasoning; and, likewise, before we can reason deductively from general truths to particular ones we must have discovered the general truths by inductive reasoning from particular facts. Thus there is a unity in all reasoning processes as there is in all mental operations. Inductive reasoning is a _synthetical_ process; deductive reasoning, an _analytical_ one. In the first we combine and build up, in the latter we dissect and separate.