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dominated by a fixed idea of any kind, is in danger of jumping from an insufficient number of special instances that favor his view to a general assertion which might be met by special instances that favor the opposite view.
A singular instance of induction unwarranted by the facts on which it is based is furnished by the comment of a recent writer on a passage which he quotes from Defoe's “Serious Reflections." The passage begins, “I have heard of a man, that, upon some extraordinary disgust which he took at the unsuitable conversation of some of his nearest relations, whose society he could not avoid, suddenly resolved never to speak any more.” This resolve, as Defoe goes on to show, the man kept, with disastrous results, nearly twenty-nine years. The comment referred to is as follows:
“ That the paragraph had reference to Defoe is evident from the opening sentence; ... 'I have heard of a man,' I know a man,' and the like, being favorite prologues of Defoe's when he was about to introduce bits of personal history.”1
The conclusion that Defoe always meant himself when he said “I have heard of a man,” “I know a man,” etc., is unwarranted.
A variety of this fallacy is that which consists in assuming a causal connection where none exists, in arguing that because one thing follows another it is caused by that other, — the fallacy technically known as post hoc, propter hoc. In the Middle Ages most people supposed that eclipses and comets caused disasters of various sorts; and even in our own day some half-educated persons believe that changes of the moon cause changes in the weather, that the equinoxes cause "equinoctial storms,” that the presence of thirteen at table causes the subsequent death of one of the number. The fallacy in question is not,
1 Thomas Wright: The Life of Daniel Defoe, chap. ii.
however, confined to the half-educated, as those who follow the course of medical and political discussions are aware. Some examples are given in a recent article by President Eliot:
Many popular delusions are founded on the commonest of fallacies this preceded that, therefore this caused that; or in shorter phrase, what preceded, caused. For example: I was sick; I took such and such a medicine and became well; therefore the medicine cured me. During the Civil War the Government issued many millions of paper money, and some men became very 1; therefore the way to make all men richer must be to issue from the Government presses an indefinite amount of paper money. ... Bessemer steel is much cheaper now than it was twenty years ago; there has been a tariff tax on Bessemer steel in the United States for the past twenty years; therefore the tax cheapened the steel. England, France, and Germany are civilized and prosperous nations; they have enormous public debts; therefore a public debt is a public blessing. He must carry Ithuriel's spear and wear stout armor who can always expose and resist this fallacy.” 1
Connection between induction and deduction.
If a gen
Since deduction uses as premisses the generalizations made by induction, it furnishes a valuable means of test
ing the validity of these generalizations by applying them to particular cases.
eralization so used turns out to be false, a new premiss may be provided by induction.
In all reasoning it is usual to combine the inductive with the deductive method; but whereas the trained rea
soner can, if he chooses, analyze his processes tion combined. of thought, the untrained reasoner goes from one method to the other without knowing what he is doing. That there is, however, no essential difference
1 Charles W. Eliot : Wherein Popular Education has Failed. The Forum, December, 1892, p. 424. See also Mill's “System of Logic,” book v. chap. v. sect. v.
Induction and deduc
between scientific and unscientific processes, Professor Huxley makes clear in the following passages :
“Scientific reasoning differs from ordinary reasoning in just the same way as scientific observation and experiment differ from ordinary observation and experiment — that is to say, it strives to be accurate; and it is just as hard to reason accurately as it is to observe accurately.
“In scientific reasoning general rules are collected from the observation of many particular cases; and, when these general rules are established, conclusions are deduced from them, just as in every-day life. If a boy says that · marbles are hard,' he has drawn a conclusion as to marbles in general from the marbles he happens to have seen and felt, and has reasoned in that mode which is technically termed induction. If he declines to try to break a marble with his teeth, it is because he consciously, or unconsciously, performs the converse operation of deduction from the general rule • marbles are too hard to break with one's teeth.'” 1
“ The vast results obtained by Science are won by no mystical faculties, by no mental processes, other than those which are practised by every one of us, in the humblest and meanest affairs of life. A detective policeman discovers a burglar from the marks made by his shoe, by a mental process identical with that by which Cuvier restored the extinct animals of Montmartre from fragments of their bones. Nor does that process of induction and deduction by which a lady, finding a stain of a peculiar kind upon her dress, concludes that soinebody has upset the inkstand thereon, differ in any way, in kind, from that by which Adams and Leverrier discovered a new planet.
“ The man of science, in fact, simply uses with scrupulous exactness the methods which we all, habitually and at every moment, use carelessly.”
1 Huxley: Introductory Science Primer.
2 Ibid. : Lay Sermons; On the Educational Value of the Natura) History Sciences.
ANTECEDENT PROBABILITY, EXAMPLE, SIGN.
The classification of arguments as deductive and inductive, though primarily useful to a student of logic, is not Three classes without value to a student of rhetoric, since it of arguments. helps him to test the validity of his own or another's reasoning. A classification more convenient for our purposes is that which distinguishes arguments according to the sources from which they come, - according as they are derived (1) from the relation of cause to effect, (2) from the resemblance which persons or things bear to one another in certain particulars or under certain aspects, (3) from the association of ideas. Arguments of the first class are called arguments from ANTECEDENT PROBABILITY; those of the second class, arguments from EXAMPLE; those of the third class, arguments from SIGN.
No form of argument is in more frequent use than the Argument from argument from ANTECEDENT PROBABILITY. probability. This argument is employed in reasoning either from the present to the future, or from the past to the present or the future.
We argue from antecedent probability that the superior skill which has enabled a base-ball nine to win successive victories will enable it to win again; that a habit (bad or good) once formed will continue; that a national peculiarity which has been shown in military affairs will be shown in civil affairs when opportunity arises. Shrewd observers of the condition of things in France in the middle of the eighteenth century argued from antecedent probability that a revolution was at
hand. Statesmen who had studied the English character and the course of events in the American colonies anticipated, long before antecedently to the actual struggle, that there would be a conflict between those colonies and the mother country. A few far-seeing Americans anticipated before Fort Sumter was fired upon that there would be an attempt to separate the slave States from the free. Any one who knew the Puritan character might have foreseen very early in the seventeenth century that if the Puritans came into power they would close the theatres. A student of English literature might have foreseen that the Elizabethan era would be characterized by the predominance of the drama; and this general probability would have been strengthened by the special probability furnished by Queen Elizabeth's liking for the theatre combined with her love of the classics. In each of these cases, the argument from antecedent probability is a means of inferring what is likely to be from what is or from what has been. The argument rests on the generally-accepted belief that certain causes tend to produce certain effects, that what Matthew Arnold calls “the stream of tendency” will continue to flow in the direction once taken.
The argument from antecedent probability is also used as a means of accounting for what has already happened. A reasoner, assuming a proposition to be true, tries to show how it probably came to be true. If a loaf of bread which had been within reach of a starving man were to disappear, an argument that the starving man was the thief might be based on knowledge of the fact that he was starving; for experience shows that a starving man is likely to lay hands on anything eatable that comes in his way. This probability existed before