But it is to be observed that the arguments from analogy give us at the best only probable truth. The degree of probability depends upon the nature and number of the resemblances upon which the conclusion is based. There must be no point of dissimilarity that would disprove the conclusion inferred.
(3) We may establish a conclusion by an array of facts. This is called inductive reasoning. We observe, for example, that A, B, C, and all other men of the past, so far as our knowledge goes, have died; and in view of these individual cases we draw the comprehensive conclusion that all men are mortal.
But this mode of reasoning, common and indispensable as it is, needs to be employed with caution. There is always danger of inferring more than the facts warrant. When the inference is based on an inadequate induction of facts, the process is called "jumping at a conclusion,"—a mistake that is frequently made. Even large inductions are not always safe. We might conclude, for instance, that, because the bulldog, hound, mastiff, setter, spaniel, terrier, and other species we have known, are accustomed to bark, therefore all dogs bark. Yet this apparently well-founded conclusion is erroneous, for there is a non-barking species in Greenland.
(4) Again, we may establish a truth by showing that it comes within an established and recognized principle. This process is known as deductive reasoning. The principle on which deductive reasoning depends is the self-evident truth that "whatever is true of the whole is true of the parts." Starting from the general truth that all men are mortal, we may conclude that A, B, and C are mortal.
The general truth that supplies the basis of deductive reasoning may be taken from various sources. Sometimes the truth is self-evident or intuitive, as the axioms that lie at the basis of mathematical reasoning. Sometimes they are truths arrived at by inductive processes. Sometimes they are maxims that have gained the assent of mankind; and again, they are the statements of an accepted philosophy, creed, code, or other recognized source of authority.
In deductive reasoning two points need particular attention: (1) the fundamental principle on which the argument is based should be well established or recognized as true; and (2) the conclusion should necessarily follow from the truth assumed in the beginning, and not embrace more than is duly warranted by it.
The general structure of an oration is determined by its object. There should be, in large measure, simplicity, unity, and progress. The language should be within the comprehension of the average hearer; the sentences, as a rule, should be brief and forcible; and the general style should be concrete rather than abstract. All parts of the oration should be bound together by the single truth and purpose at which the orator aims. The arguments should not be abstruse but clear and striking. Irrelevant matter of every kind, no matter how brilliant in itself, should be excluded; and every fact and principle should be scrupulously correct. Understatement is better than overstatement. The orator should continually advance toward his conclusion; the auditor should feel himself borne along not on a circling eddy but on the bosom of a full, strong current of thought and feeling.
It was Cicero who said that the orator should know everything. However desirable such attainments might be, they are no longer possible; but the orator should have a wide range of culture and experience. This is necessary to give breadth and proportion to his outlook upon the world. In addition to this general culture he should, as far as possible, be master of his subject; and to this end he should bestow upon his discourse careful and even laborious preparation. Without these requisites the orator is apt to prove uninteresting, inconclusive, and unsuccessful.
