Or, again:—
I. (Major premise)—Rattlesnakes frequently bite when enraged, and their bite is poisonous.
II. (Minor premise)—This snake before me is a rattlesnake; therefore
III. (Conclusion)—This snake before me may bite when enraged, and its bite will be poisonous.
The average person may be inclined to object that he is not conscious of going through this complicated process when he reasons about sparrows or rattlesnakes. But he does, nevertheless. He is not conscious of the steps, because mental habit has accustomed him to the process, and it is performed more or less automatically. But these three steps manifest in all processes of deductive reasoning, even the simplest. The average person is like the character in the French play who was surprised to learn that he had "been talking prose for forty years without knowing it." Jevons says that the majority of persons are equally surprised when they find out that they have been using logical forms, more or less correctly, without having realized it. He says: "A large number even of educated persons have no clear idea of what logic is. Yet, in a certain way, every one must have been a logician since he began to speak."
There are many technical rules and principles of logic which we cannot attempt to consider here. There are, however, a few elementary principles of correct reasoning which should have a place here. What is known as a "syllogism" is the expression in words of the various parts of the complete process of reasoning or argument. Whately defines it as follows: "A syllogism is an argument expressed in strict logical form so that its conclusiveness is manifest from the structure of the expression alone, without any regard to the meaning of the term." In short, if the two premises are accepted as correct, it follows that there can be only one true logical conclusion resulting therefrom. In abstract or theoretical reasoning the word "if" is assumed to precede each of the two premises, the "therefore" before the conclusion resulting from the "if," of course. The following are the general rules governing the syllogism:—
I. Every syllogism must consist of three, and no more than three, propositions, namely (1) the major premise, (2) the minor premise, and (3) the conclusion.
II. The conclusion must naturally follow from the premises, otherwise the syllogism is invalid and constitutes a fallacy or sophism.
III. One premise, at least, must be affirmative.
IV. If one premise is negative, the conclusion must be negative.
V. One premise, at least, must be universal or general.
VI. If one premise is particular, the conclusion also must be particular.