In the preceding chapters we have seen that in the group of mental processes involved in the general process of Understanding, there are several stages or steps, three of which we have considered in turn, namely: (1) Abstraction; (2) Generalization or Conception; (3) Judgment. The _fourth_ step, or stage, and the one which we are now about to consider, is that called Reasoning.
_Reasoning_ is that faculty of the mind whereby we compare two Judgments, one with the other, and from which comparison we are enabled to form a third judgment. It is a form of indirect or mediate comparison, whereas, the ordinary Judgment is a form of immediate or direct comparison. As, when we form a Judgment, we compare two concepts and decide upon their agreement or difference; so in Reasoning we compare two Judgments and from the comparison we draw or produce a new Judgment. Thus, we may reason that the particular dog “Carlo” is an animal, by the following process:
(1) _All_ dogs are animals; (2) Carlo is a dog; therefore, (3) Carlo is an animal. Or, in the same way, we may reason that a whale is not a fish, as follows:
(1) _All_ fish are cold-blooded animals; (2) A whale is _not_ a cold-blooded animal; therefore, (3) A whale is _not_ a fish.
In the above processes it will be seen that the third and final Judgment is derived from a comparison of the first two Judgments. Brooks states the process as follows: “Looking at the process more closely, it will be seen that in inference in Reasoning involves a comparison of relations. We infer the relation of two objects from their relation to a third object. We must thus grasp in the mind two relations and from the comparison of these two relations we infer a third relation. The two relations from which we infer a third, are judgments; hence, Reasoning may also be defined as the process of deriving one judgment from two other judgments. We compare the two given judgments and from this comparison derive the third judgment. This constitutes a single step in Reasoning, and an argument so expressed is called a _Syllogism_.”
The _Syllogism_ consists of three propositions, the first two of which express the grounds or basis of the argument and are called the _premises_; the third expresses the inference derived from a comparison of the other two and is called the _conclusion_. We shall not enter into a technical consideration of the Syllogism in this book, as the subject is considered in detail in the volume of this series devoted to the subject of “Logic.” Our concern here is to point out the natural process and course of Reasoning, rather than to consider the technical features of the process.
Reasoning is divided into two general classes, known respectively as (1) _Inductive Reasoning_; (2) _Deductive Reasoning_.
_Inductive Reasoning_ is the process of arriving at a general truth, law or principle from a consideration of many particular facts and truths. Thus, if we find that a certain thing is true of a great number of particular objects, we may infer that the same thing is true of _all_ objects of this particular kind. In one of the examples given above, one of the judgments was that “all fish are cold-blooded animals,” which general truth was arrived at by Inductive Reasoning based upon the examination of a great number of fish, and from thence assuming that _all_ fish are true to this general law of truth.
_Deductive Reasoning_ is the reverse of Inductive Reasoning, and is a process of arriving at a particular truth from the assumption of a general truth. Thus, from the assumption that “all fish are cold-blooded animals,” we, by Deductive Reasoning, arrive at the conclusion that the particular fish before us must be cold-blooded.
Inductive Reasoning proceeds upon the basic principle that “_What is true of the many is true of the whole_,” while Deductive Reasoning proceeds upon the basic principle that “_What is true of the whole is true of its parts_.”
Regarding the principle of _Inductive Reasoning_, Halleck says: “Man has to find out through his own experience, or that of others, the major premises from which he argues or draws his conclusions. By induction, we examine what seems to us a sufficient number of individual cases. We then conclude that the rest of these cases, which we have not examined, will obey the same general law. The judgment ‘All men are mortal’ was reached by induction. It was observed that all past generations of men had died, and this fact warranted the conclusion that all men living will die. We make that assertion as boldly as if we had seen them all die. The premise, ‘All cows chew the cud,’ was laid down after a certain number of cows had been examined. If we were to see a cow twenty years hence, we should expect to find that she chewed the cud. It was noticed by astronomers that, after a certain number of days, the earth regularly returned to the same position in its orbit, the sun rose in the same place, and the day was of the same length. Hence, the length of the year and of each succeeding day was determined, and the almanac maker now infers that the same will be true of future years. He tells us that the sun on the first of next December will rise at a given time, although he cannot throw himself into the future to verify the conclusion.”