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which receive the most simple and satisfactory explana tion from supposing that the sun is at rest, and is the centre of those motions. The latter phenomena would therefore serve as instantiæ crucis, by which the superior credibility of the Copernican system was fully evinced "*
$ 296. Of combined or accumulated arguinents. When a proposition in geometry is given to be demonstrated, it sometimes happens that two or more solutions may be offered leading to the same end. The theorem or the problem is one and the same, as also the conclusion; but there may be more than one train of reasoning, more than one series of intermediate steps, connecting the proposition which is to be investigated with the result. But as the conclusion in each of these different cases is certain, it does not strengthen it, although it may gratify curiosity to resort to a different and additional process.
It is not thus in moral reasoning. The great difference between the two kinds of reasoning, as before observed, is not so much in the mental process as in the subjects about which they are employed. Now as the subjects in moral reasoning are not of a purely abstract nature, and are, therefore, often attended with uncertainty, our belief, when we arrive at the conclusion, is not always of the highest kind. More frequently it is some inferior degree of probability. Hence, in any moral inquiry, the more numerous the series of arguments which terminates in a particular conclusion, the stronger will be our belief in the truth of that conclusion.
Thus we may suppose a question to arise, Whether the Romans occupied the island of Great Britain at some period previous to the Saxon conquest? In reference to this inquiry a number of independent arguments may
be brought forward : (1.) The testimony of the Roman historians; (2.) The remains of buildings, roads, and encampments, which indicate a Roman origin; (3.) The coins, urns, &c., which have been discovered. Although these arguments are independent of each other, they all
* See Works of John Playfair, Esq., Edinburgh edition, vol. ii., p. 105
bear upon the same conclusion; and, being combined together, they very essentially increase the strength of our belief.
PRACTICAL DIRECTIONS IN REASONING.
$ 297. Rules relating to the practice of reasoning. VARIOUS directions have been given by writers on Logic (which, it may be remarked here, is only another name for whatever concerns the nature, kinds, and applications of reasoning), the object of which is to secure the more prompt, accurate, and efficient use of the reasoning power. It is but natural to suppose that some of these dialectical rules are of greater, and others of less value. Such as appeared to be of the least questionable importance are brought together and explained in this chapter; nor will this occasion any surprise when it is recollected that it has been the object of this work throughout, not only to ascertain what the mental operations are, but, by practical suggestions from time to time, to promote what is of a good, and prevent what is of a hurtful tendency in such operations.
The directions now referred to have of course a more intimate connexion with Moral than with Demonstrative reasoning ; but this is a circumstance which enhances rather than diminishes their worth. The occasions which admit and require the application of moral reasoning, being inseparable from the most common occurrences and exigences of life, are much more numerous than those of demonstrative reasoning.
Ø 298. Of being influenced in reasoning by a love of the truth.
(I.) The first direction in relation to reasoning which will be given, concerns the feelings with which it is proper to be animated. It is this. In all questions which adinit of discussion, and on which we find ourselves at
variance with the opinions of others, we are to. m.ike truth our object. The opposite of a desire of the truth is a wish to decide the subject of dispute in one way or another, independently of a just consideration of the evidence. The foundation of such a preference of one result to another is in general the prejudices of interest and passion ; and these are the great enemies of truth. Whenever we are under their influence, we form a different estimation of testimony and of other sources of evidence from what we should do under other circumstances; and at such times they can hardly fail to lead us to false results.—This rule is important on all occasions of reasoning whatever, but particularly in public debate; because at such times the presence of others and the love of victory combine with other unpropitious influences to. induce mien to forget or to disregard the claims which truth is always entitled to enforce. $ 299. Care to be used in correctly stating the subject of discussion.
(II.) Another rule in the prosecution of an argument is, that the question under debate is to be fairly and correctly stated. The matter in controversy may be stated in such a way as to include, in the very enunciation of it, something taken for granted, which must necessarily lead to a decision in favour of one of the opponents. But this amounts to begging the question, a species of fallacy or sophism upon which we shall again have occasion to remark.–Sometimes the subject of discussion is stated so carelessly, that the true point at issue is wholly left out. It may be proper, therefore, in many cases to adopt the practice of special pleaders, and first to ascertain all the points in which the opponents agree, and those in which they differ. And then they can hardly fail of directing their arguments to what is truly the subject of contention.
In order that there may not be a possibility of misunderstanding here, dialecticians should aim to have clear ideas of everything stated in the question which has an intimate connexion with the point at issue. Subordinate parts of the question, and even particular words, are to be examined. If, for instance, the statement affirm or deny anything in regard to the qualities or properties of mate
rial bodies, it is incumbent upon us to possess as clear ideas as possible both of the object in general and of those properties or qualities in particular. Similar remarks will apply to other subjects of inquiry of whatever kind.
Ø 300. Consider the kind of evidence applicable to the subject. (III.) As one subject clearly admits of the application of one species of evidence, while another as clearly requires evidence of a different kind, we are thence ena. bled to lay down this rule, viz., We are to consider what kind of evidence is appropriate to the question under discussion.
When the inquiry is one of a purely abstract nature, and all the propositions involved in the reasoning are of the same kind, then we have the evidence of Intuition or intuitive perception; and the conclusion, for reasons already mentioned, is certain.—In the examination of the properties of material bodies, we depend originally on the evidence of the Senses, which gives a character and strength to our belief according to the circumstances under which the objects are presented to them. In judging of those facts in events and in the conduct of men which have not come under our own observation, we rely on Testimony. This source of belief causes probability in a greater or less degree, according as the testimony is from one or more, given by a person who understands the subject to which it relates, or not, &c.—And again, some subjects admit of the evidence of Induction, and in respect to others we have no other aids than the less authoritative reasonings from Analogy. In other cases, the evidence is wholly made up of various incidental circumstances which are found to have relation to the sub. ject in hand, and which affect the belief in different degrees and for various causes.
And hence, as the sources of belief, as well as the belief itself, have an intimate connexion with the subject before us, they ought to be taken into consideration. The evidence should be appropriate to the question. But if the question admit of more than one kind of evidence, then all are entitled to their due weight.
301. Reject the aid of false arguments or sophisms (IV.) There is a species of false reasoning which we call a SOPHISM. A sophism is an argument which contains some secret fallacy under the general appearance of correctness. The aid of such arguments, which are calculated to deceive, and are, in general, inconsistent with a love of the truth, should be rejected.
(1.) IGNORATIO ELENCHI, or misapprehension of the question, is one instance of sophism. It exists when, from some misunderstanding of the terms and phrases that are employed, the arguments advanced do not truly apply to the point in debate. It was a doctrine, for instance, of some of the early philosophic teachers of Greece, that there is but one principle of things. Aristotle, understanding by the word principle what we commonly express by the word ELEMENT, attempted to show the contrary, viz., that the elements are not one, but many, thus incurring the imputation of IGNORATIO ELENCHI; for those who held the doctrine which was thus subjected to his animadversion, had reference, not to the form, but the cause of things; not to any doctrine of elementary material particles, but to the intellectual origin, the creative mind, the Supreme Being, whom, as the PRINCIPLE (that is, as the beginning and the support of things), they maintained to be one." (2.) PETITIO PRINCIPII
, or begging of the question, is another instance of sophism. This sophism is found whenever the disputant offers in proof of a proposition, the proposition itself in other words. The following has been given as an instance of this fallacy in reasoning : A person attempts to prove that God is eternal by maintaining that his existence is without beginning and without end. Here the proof which is offered, and the proposition itself which is to be proved, are essentially the same.- When we are told that opium causes sleep beca'use it has a soporific quality, or that grass grows by mians of its vegetative power, the same thing is repeated in other terms.—This fallacy is very frequently practised; and a little care in detecting it would spoil inany a fine saying, as well as deface many an elaborate argument
La Logique ou l'Art de Penser (Port Royale), pt. iii., chap xix