to microscopic or chemical analysis. Adams and Leverrier proved the existence of Neptune as the only possibility of reconciling the aberrations of Uranus with the law of gravitation.

66. This question of certainty and probability is not speculative, it has great practical value. Trials at law, involving the property, reputation, and even the lives of men and women, are decided by proofs which are not in themselves perfectly conclusive, but only probable. The direct testimony of witnesses may be erroneous or false; circumstantial evidence, apparently conclusive, may have more than one flaw. Not even the so-called confession of an accused person is perfectly conclusive: instances have occurred in which an innocent person has incriminated himself for peculiar reasons. It is precisely because of this element of uncertainty that law-trials exhibit such perplexities. For instance, in the celebrated Kenniston case, see $57, the Kennistons would probably have been convicted but for Webster's ingenuity in cross-examining the principal witness, Goodrich. The evidence against them seemed so strong that several lawyers declined to undertake the defence. Webster was induced to undertake it only at the last moment, and in response to a strong personal appeal.

Matters of legislation and finance also rest upon arguments that are far from conclusive. The utmost that can be said in favor of certain proposed measures is that they will probably work well in one country because they have worked fairly well in another. But until they have been actually tested for a number of years we cannot be sure.

GENERAL AND PARTICULAR.

67. In argumentation we may start from a general and infer a particular; conversely, we may start from one or more particulars and infer a general. Thus, in the cases given in the beginning of § 65, we start from a particular

phenomenon (peculiar form of growth, flag flying) and infer a general cause (human agency), or an antecedent general condition (Congress in session). On the other hand, when we observe the thermometer at zero we know that the temperature is very low, and we infer from this low temperature (as a cause) that standing water must be freezing (particular phenomenon). If, at the trial of A. for the murder of B., witnesses testify that A. had frequently expressed intense hatred of B., we infer from this hatred (as a motive) the probability, greater or less, that he may have committed the murder (particular act). In criminal trials it is usually needful to establish some motive or inducement for committing the offence in question, e. g., jealousy, revenge, desire of money, etc. This necessity is due to the popular assumption, see § 69, that a man does not commit a crime without some special motive. Proving the existence of the motive does not directly prove the accused to be guilty, but merely removes the improbability arising from the absence of all motive. See remarks on Webster's defence of the Kennistons, § 57.

Observe how these four simple cases exemplify the statement (end of § 63) that the two methods of classification cross each other.

It is believed that this presentation of arguments according to the principle of Certainty-Probability and General-Particular will be more intelligible to the young student than the usual classification, viz., Antecedent Probability, Sign, and Example. Antecedent Probability corresponds in the main to the inference from general to particular; Sign, to the inference from particular to general. Example will be treated later, in connection with Analogy, §§ 71, 73.

A few terms, frequently used in argumentation, need separate explanation.

K

INDUCTION DEDUCTION.

68. Induction is, loosely speaking, the process of inferring a general from particulars. But in science the term is employed in a very restricted sense. We speak of scientific induction only when the inference amounts to a general law, or a general cause; perhaps also when the inference amounts to what may be called an empiric general fact. Thus, the law of gravitation, quoted in § 55, is an induction of the highest order. When we see streams and ponds frozen, we infer that the temperature has been below 32°. By Empiric Fact is meant that two phenomena are always found associated, so that when we see one we expect to see the other, although no direct causal relation between the two has been established. Thus, in zoology, the presence of occipital condyles is uniformly associated with the presence of mammæ and warm red blood, and this general fact is treated as a quasi induction. Hence, when the remains of a new animal are discovered, exhibiting such condyles, the zoologist infers, by deduction, that the animal belonged to the class Mammalia.

Deduction is the converse of induction. Given a law, or a cause, we infer particular phenomena previously unknown. Thus Franklin, having proved by induction that electricity and lightning are identical, made the deduction. that lightning might be diverted from buildings by using a metal rod. Newton, having established the law of gravity (55), made the deduction that the attractive force exerted by a sphere equal to the earth in size upon a body outside of it is the same as if the whole mass of the earth were contracted to a point at its centre. That is, the centre of the earth is the starting-point from which to

measure.

Our confidence in scientific inductions and deductions rests in our ability to verify them by repeated experiments. Newton's law of gravity is put to a practical test every day

in every astronomical calculation. His deduction relating to the centre of the earth was tested by numerous observations upon the movements of the moon. Every lightningrod is a verification of Franklin's deduction.

But deductions from empiric facts are never quite certain in the strictest scientific sense. Fresh discoveries may reveal exceptions. Thus, until 1824, it was supposed that the circulation of the blood was always in one definite and invariable direction :

But in that year M. Von Hasselt, happening to examine a transparent animal of the class of Ascidians, found to his infinite surprise that after the heart had beat a certain number of times it stopped and then began beating the opposite way, so as to reverse the course of the current, which returned by and by to its original direction.-HUXLEY (v.), p. 86.

Another empiric fact is the absence of teeth in birds. This is so constant that until 1872 it was supposed that teeth and feathers never occurred in the same animal. A naturalist, discovering remains of an unknown animal with teeth, would have inferred, prior to 1872, that it could not belong to the class of birds. But in that year Prof. O. C. Marsh discovered in Western Kansas fossil remains of birds with clearly defined teeth, and proposed to constitute them as a sub-class, the Aves Dentatæ or Odontornithes, in three distinct orders.

A further empiric fact, which now promises perhaps to become a genuine induction, is the connection between the Aurora Borealis, magnetic storms, sun-spots, and the movements of Jupiter, Saturn, Venus, and Mars.

ASSUMPTION.

69. Much, perhaps the greater part, of argumentation rests ultimately upon assumption. In its highest form an assumption is called an axiom, i. e., a self-evident proposition; e. g., that two things equal to the same thing are

equal to each other, that the whole is greater than a part, that matter is indestructible.

Many of the assumptions that we hear in every-day life are the expression of the common experience of men, and vary greatly in their probability. Thus, the proposition that all men are mortal, or the proposition that every man is a mixture of good and bad, that every man is liable to err, etc. Such assumptions are usually called maxims. But the popular belief that "murder will out" (see § 56) is scarcely a maxim.

It is interesting to study the history of one great political assumption. The opening sentence of Lincoln's Gettysburg address runs thus:

Four score and seven years ago our fathers brought forth on this continent a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal.

The assertion of the Declaration of Independence, that all men were created free and equal, was certainly a much debated proposition until the great war of 1860-65. But now, for American affairs at least, it is a political and legal axiom. Yet, for many other parts of the world, notably Africa and some of the East Indies, it is still an open question.

Our belief in what we read in books and newspapers is largely an assumption that the writers are truthful and know what they write of. Were we to demand proof of everything that we read, we should come to a stand-still.

In law there are various assumptions, sometimes called presumptions, e. g., the rule that a child under seven years of age is incapable of committing crime. This particular presumption is denominated" conclusive," i. e., it does not admit of any argument to the contrary. But the presumption that a man accused of crime is innocent until proved guilty is not at all conclusive. It merely amounts to saying that the burden of proof rests upon the prosecution.

In popular speech presumption is used loosely to desig