Pagina-afbeeldingen
PDF
ePub

knowledge, the other can properly be said to produce in most cases only judgment or opinion.—But the probabil. ity of such judgment or opinion may sometimes arise so high as to exclude all reasonable doubt. And hence we then speak as if we possessed certainty in respect to subjects which admit merely of the application of moral reasoning. Although it is possible that there may be some difference between the belief attendant on demonstration and that produced by the highest probability, the effect on our feelings is at any rate essentially the same. A man who should doubt the existence of the cities of London and Pekin, although he has no other evidence of it than that of testimony, would be considered hardly less singular and unreasonable than one who might take it into his head to doubt of the propositions of Euclid.It is this very high degree of probability which we term moral certainty.

$ 291. Of reasoning from analogy. MORAL REASONING admits of some subordinate divisions; and of these, the first to be mentioned is reasoning from analogy.—The word analogy is used with some vagueness, but in general denotes a resemblance, either greater or less.-Having observed a consistency and uniformity in the operations of the physical world, we are naturally led to presume that things of the same nature will be affected in the same way, and will produce the same effects; and also that the same or similar effects are to be attributed to like causes. ANALOGICAL REASONING, therefore, is that mental process by which unknown truths or conclusions are inferred from the resemblance of things.

The argument by which Sir Isaac Newton establishes the truth of universal gravitation is of this sort. He proves that the planets in their revolutions are deflected towards the sun in a manner precisely similar to the deflection of the earth towards the same luminary; and also that there is a similar deflection of the moon towards the earth, and of a body projected obliquely at the earth's surface towards the earth's centre. Hence he infers by analogy, that all these deflections originate from the same cause, or are governed by one and the same law, viz., the power of gravitation..

This method of reasoning is applicable to the inquiry, Whether the planets are inhabited ? and furnishes the sole ground for the indulgence of such a supposition. We observe a resenblance in certain respects between Mars, Jupiter, Saturn, and other planets, and the Earth They all revolve around the sun as the Earth does, and all derive light from that source. Several of thern are ascertained to revolve on their axis, and, consequently, must have : succession of day and night. Soine of ther: have moons, and all are subject to the law of gravitatior. From these various similitudes we draw the conclusion by analogy, that those planets must be inhabited like the Earth.

There are a variety of subjects, both speculative and practical, in respect to which we may reason in this way, and sometimes with considerable satisfaction. And, among others, this method of reasoning finds a place in the arguments of persons in the practice of the law. An attorney, for instance, advocates a case which does not fall within the provisions of existing statutes, and for which he finds in his authorities no exact precedent. He is therefore under the necessity of ascertaining, as far as possible, the analogy or resemblance between this case and others which have been made the subject of judicial decisions; and this analogy he makes the basis of his argument.

$ 292. Caution to be used in reasoning from analogy. The remark remains to be made, that much care is necessary

in arguments drawn from this source, especially in scientific investigations; and they are in all cases to be received with some degree of distrust. The ancient anatcais's are an instance of precipitate reasoning from analogy. Being hindered by certain superstitions from dissecting the bodies of men, they endeavoured to obtain the information they wanted by the dissection of those animals whose internal structure was supposed to coine nearest to that of the human body. In this way they were led into a variety of mistakes, which have been de

X

tected by later anatomists. It does not follow, because things resemble each other in a multitude of particulars, that this resemblance will be found in all others; and we are, therefore, always to consider ourselves in danger of pushing the supposition of similitude too far.

The proper use of analogical reasoning seems to be, in all scientific inquiries, merely to illustrate and confirm truths which are susceptible of proof from other sources of evidence, either by casting a direct additional light or by answering objections. A happy instance of this use of it is the work of Bishop Butler, entitled, “ The Analogy of Religion, natural and revealed, to the Constitution and Course of nature.”—It is not the object of the writer to prove the truth of religion, but to answer some objections which may be brought against its practical details and its principles by those who, while they object to the Bible, still maintain the existence of a God. And this he does by proving that the same objections exist to the providence of God in the natural world. There is an Analogy or resemblance in the two, viz., between the administration of God as made known in the Bible, and his administration of things as made known in the natural world; and if the objections which are brought forward will reject him from the authorship of what we term Religion, they will dethrone him also, for the same reason, from all authorship and direction in the ordinary economy of nature.

§ 293. Of reasoning by induction. We now come to another method of moral reasoning, viz., by induction. Inductive reasoning is the inferring of general truths from particular facts that have fallen under our observation. Our experience teaches us that nature is governed by uniform laws; and we have a firm expectation (whether it be an original principle of our constitution, or whatever may be the origin of it) that events will happen in future, as we have seen them happen in times past. With this state of mind we are prepared to deduce inferences by induction.

When a property has been found in a number of subjects of the same kind, and nothing of a contradictory

nature appears, we have the strongest expectation of finding the same pr sperty in all the individuals of the same class; in other words, we come to the conclusion that the property is a general one. Accordingly, we apply a magnet to several pieces of iron; we find in every instance a strong attraction taking place; and we conclude, although we have made the experiment with only a small number of the masses of iron actually in existence, that it is a property of iron to be thus affected by that substance, or that all iron is susceptible of magnetical attraction. This is a conclusion drawn by induction.

The belief which attends a well-conducted process of inductive reasoning bears a decided character; it is moral probability of the highest kind, or what is sometimes termed moral certainty ; and is at least found to be sufficient for all practical purposes. We obtain all the

eral truths relating to the properties and laws of material objects in this way.

And we thus not only acquire a knowledge of material objects, but apply the same inductive process also in the investigation of laws which govern the operations of the mind. It is by experience, or observing what takes place in a number of individuals, that we are able to infer the general law of association, viz., when two or more ideas have existed in the mind in immediate succession, they are afterward found to be mutually suggested by each other. It is the same in ascertaining other general laws of the mind.

$ 294. Of the caution necessary in inductive processes. Reasoning in this method requires the exercise of caution no less than by analogy. It is especially liable to prove fallacious, whenever our investigations have been marked with impatience, and our judgments are formed on a very small number of facts.

When the number of examined instances is large, and the results are uniform, the conclusion amounts to moral certainty. But when the number of such instances is small, and the results are not altogether uniform, the judgments formed wil. possess a greater or less degree of probability, varying with circumstances.--And espe

cially is the mind left in a state of vacillation and uncertainty, when results have repeatedly occurred under such circumstances as to leave us at liberty to ascribe them to a diversity of causes. In such cases we find ourselves recluced to the necessity of resorting to what writers on this subject have termed INSTANTIÆ CRUCIS.—The impor tant and decisive method in Inductive Reasoning of in STANTIÆ CRUCIS was first laid down in the Novum Organum of Bacon, and has recently been happily illustrated in Playfair's View of the Progress of Mathematical and Physical Science in the following terms. $ 295. Of instances or experiments in inductive reasoning termed in

stantiæ crucis. “When the understanding is placed in equilibrio, as it were, between two or more causes, each of which accounts equally well for the appearances, as far as they are known, nothing remains to be done but to look out for a fact which can be explained by one of these causes and not by the other; if such a one can be found, the uncertainty is removed, and the true cause is determined. Such facts perform the office of a cross, erected at the separation of two roads, to direct the traveller which he is to take, and on this account Bacon gave them the name of instantiæ crucis.

Suppose that the subject inquired into were the motion of the planets, and that the phenomena which first present themselves, or the motion of these bodies in longitude, could be explained equally on the Ptolemaic and the Copernican system, that is, either on the system which makes the Earth, or that which makes the Sun, the centre of the planetary motions, a cautious philosopher would hesitate about which of the two he should adopt; and, notwithstanding that one of them was recommended by its superior simplicity, he might not think himself authorized to give to it a decided preference above the other. If, however, he consider the motion of these bodies in latitude, that is to say, their digressions from the plain of the ecliptic, he will find a set of phenomena which cannot be reconciled with the supposition that the Earth is the centre of the planetary motions, but

[ocr errors]
« VorigeDoorgaan »