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ILLICIT ASSUMPTION OF PREMISES (PETITIO
PRINCIPII) 1. Of the Nature and Several Forms of this
$ 154. This fallacy may occur in various ways, and it would therefore be an endless task to enumerate or classify all its different forms; nor would there be any advantage in doing so. There are, however, several forms of the fallacy that, on account of their frequent occurrence and their powerful influence over the minds of men, demand a particular consideration, and to these our attention will be directed.
$ 155 (1). ILLICIT GENERALIZATION.— The most important of these, which may be called the Fallacy of Illicit Generalization, consists in the use of a universal proposition in cases where the corresponding particular proposition is alone admissible. This fallacy is one of the most common and formidable, not only in popular discourses, but in more pretentious
works on Politics and Morality; for almost all the wisdom of common sense is embodied in this sort of propositions, i. e., particular propo- . sitions assumed to be universal. Such propositions may, indeed, be used with profit by men of sense in practical affairs; as, in general, when a question presents itself it is easy to perceive whether the principle should be applied or not; or, if a mistake be made, it is corrected by experience; but the masses of men are easily misled by them. Hence they serve well for rhetorical purposes; for the hearer, unless of a critical mind, will in general accept them without hesitation.
$ 156. COMMONPLACES.–The most important cases of this fallacy occur in the use of Commonplaces; by which is meant, opinions current among men generally, or particular classes of men, and used as premises for reasoning. These are commonly founded upon some truth which they purport to express, and to which they more or less nearly approximate; so that there is here, as“ in all things evil, a soul of truth.” But they are hardly ever universally true; and therefore to assume them as universals is illicit.
1 Hence Bacon, as a useful rhetorical device, recommends the preparation of tables of Commonplaces, of which he gives an example in his De Augmentis ; wherein should be arranged, for the use of speakers and writers, in parallel columns, arguments pro and con, or theses and anti-theses, on all questions of general interest.
$ 157. POPULAR PROVERBS.-Of these commonplaces, the most striking examples are furnished by popular proverbs; and of these, as illustrating precisely the nature of such maxims, two may be cited that, in their literal expression, are contradictory, but, as maxims go, may both be said to be true, i. e., they are each true in certain cases, but neither universally. They are the old adages, “ Never put off till to-morrow what you can as well do today" and “Never do to-day what you can as well put off till to-morrow"; the first of which points out the danger of procrastination, the latter, the danger of committing ourselves before necessity requires. It may be readily seen that, according to circumstances, either of these may serve as a useful hint for conduct; but, in using it, the caution of the nautical philosopher is to be observed, that“ the bearing of the observation lies in the application of it.”
$ 158. LEGAL MAXIMS.- Another striking illustration of the same class of propositions is furnished by what are called the maxims of the law; which, in general, are true only as particular propositions, i. e., only in particular cases,
but are habitually spoken of by legal writers as “ first principles," analogous to the maxims of science; though every competent lawyer is familiar with the fact that they admit of numerous exceptions. A very large proportion of the so-called principles of the law, and of the rules founded upon them, are of precisely this nature, i. e., admit of exceptions, and are, therefore, true only as particular propositions. And it is also a fact that many of these principles and rules are opposed by others, equally approved, that are contradictory to them. Hence, if we regard bulk only, the greater part of the law might be readily and advantageously arranged in a table of contradictory commonplaces,-i.e., a collection of theses and anti-theses, -as sug. gested by Bacon in the De Augmentis; wherein, under each topic, one column should represent the one side and the other, the other, of the various questions that may arise in litigation. The cases might also be arranged in the same way.
The above examples are all cases of illicit generalization, and will serve to show how widespread is the use of this particular form of illicit assumption of premise. And, it may be added, such is the lack of critical acumen in the generality of mankind, that the fallacy is seldom detected, and consequently it constitutes the most powerful of rhetorical devices.
$ 159 (2). OF THE FALLACY OF Non CAUSA PRO CAUSA.-Another form of the Fallacy of Illicit Assumption of Premise is presented by the fallacy called “Non causa pro causa" ; which is also called the fallacy of “ Post hoc ergo propter hoc." It consists in the illicit assumption that an event preceding another event is the cause of the latter, as, a change in the moon is the cause of a change in the weather; or that the fact of thirteen dining together is the cause of any accident that may happen to any one of them; or that the Dog Star is the cause of heat. This is, indeed, one
, of the most familiar of fallacies in political arguments, where it is common to argue that the condition of the country, whether good or bad, is caused by some particular policy, as, e. g., where it is argued alternately, according to vicissitudes of events, by the one party that a prosperous, by the other that a depressed, condition of affairs is caused by the tariff or other political measure.'
" It will be observed that there are some differences of opinion among logicians as to this fallacy. A distinction is made between what is called the causa essendi and the causa cognoscendi ; or between the cause of an event and the cause of our knowing it. These may coincide, as, e..., when from the fact of its raining in the night we infer that the ground will be wet in the morning; where the rain is both the causa essendi and the causa cognoscendi. But, when, from finding the ground wet in the morning, we infer that it rained during