points where there is such resemblance in many. In the use of this species of argument, it becomes, then, of the utmost importance to bear in mind both that the most similar things differ in some respects, and perhaps in that very point on which the argument in a given case depends; and, also, that the most dissimilar things may have some properties or relations in common, and may therefore furnish foundations for valid reasoning.

The decisive test of the soundness of all arguments founded on resemblance, is furnished in the inquiry, Do the particulars of resemblance owe their existence to the same cause? As the whole force of examples as arguments rests on the sameness of the cause, or of the law or general attribute in the proof and the conclusion on which the classification depends, the detection of this cause, where possible, will ever discover the validity or invalidity of the example as an argument. Just so far as there remains a doubt of the sameness of the cause or law, so far must there be weakness in the argument.

§ 155. While all simple arguments may be referred to some one of the foregoing classes, many complex arguments partake of the nature of two or more; their force in reasoning is consequently modified in reference to the respective character of the classes of arguments of the nature of which they partake.

What is often called a priori reasoning not unfrequently includes in itself not only an antecedent-probability argument, but also a sign or an example. From the falling of the barometer we infer a priori that there will be a change of the weather; not because we suppose the fall of the mercury to be the cause of the change, but because it is the sign of the existence of the cause. We in this case, in truth, first argue by a sign, to the existence of a cause, and then by an antecedent-probability argument, to its effect, namely, a change of the weather. In the argument in "the Good

ridge case," before referred to, § 140, several circumstances are advanced as signs in proof of a cause or motive to feign a robbery; from which cause, thus proved, the inference is that the prosecution was groundless.

Lord Chatham, in his speech "on removing the troops from Boston," argues the continued and determined resistance of the Americans to an arbitrary system of taxation from the spirit of liberty which animated them in common with all Englishmen; and the existence of this spirit is proved by an example the proceedings of the General Congress at Philadelphia. This would ordinarily be called an a priori argument, inasmuch as the force of it rests mainly on the existing cause to produce the continued resistance. But an example is introduced to prove the existence of the cause, and the intermediate step of the argument, the cause itself, is not expressed but only implied.

In the same speech we have another form of the combination of the antecedent-probability argument with the example. The example is introduced, not, as in the other case, to prove the antecedent-probability argument itself, but to confirm it as proof of the main proposition. The speaker exemplifies the working of that spirit of liberty in the effectual opposition to "loans, benevolences, and ship-money in England," in the procuring of "the bill of rights," etc. The reasoning, as a whole, is a priori, but is complex, consisting of an antecedent-probability argument and examples.

race.

By an a priori argument, the fact of a revelation from Heaven is inferred from the general corruption of the human The argument consists of an antecedent-probability argument - the determination of God to do all that is necessary to effect the recovery of the race and of a sign, the corruption of the race, to prove the necessity of such an interposition by revelation.

A posteriori reasoning, also, often includes arguments of different classes. From the migration of birds to the north, we infer that some of the various effects of spring have

appeared in the place of their hibernation. From the migration of birds, as a sign, we infer the return of warm weather as its cause; and from this we infer again, by an antecedentprobability argument, the usual effects of the return of spring.

While both a priori and a posteriori reasoning thus often contain arguments of two or more classes, there is yet an obvious distinction between them. In the former, the antecedent-probability argument is the one on which the force of the reasoning mainly depends; in the latter, the sign or the example is the prominent argument.

The analysis of complex arguments will often discover the precise amount of validity due to them. It will disclose also the point where the sophistry of a suspected proof

enters.

Testimony and authority, also, often combine arguments of different species, and are themselves frequently combined together in the same process of reasoning.

What is often called reasoning from experience, is distinguished from other species of reasoning only by the source from which the arguments are derived.

The argument from progressive approach, so called, is but a species of induction, in which we argue from the increase or diminution in the effect according as a particular cause is increased or diminished in several examples, to the perfect completeness, or the entire removal of the effect when the cause is perfectly operative or wholly removed: e. g., if we put a ball in motion on a rough surface, its motion soon ceases; on a smoother surface, its motion is proportionally prolonged: hence, we infer that if there were no resistance at all the motion would be perpetual. A sophistical use of this argument has been made by some enthusiastic advocates of temperance. They have assumed that disease and death are the consequence exclusively of a corrupt constitution inherited from parents who have violated the laws of health, or of a transgression of those laws by the individual himself.

They then urge the facts that temperance and correct regimen promote health and long life, just in proportion as the constitution is free from original corruption and the laws of health are observed. They hence infer that a perfect and universal observance of the laws of health will in time purify the stock itself, the human constitution will be restored to its perfect state, and disease and death will disappear.

§ 156. EMPIRICAL proofs never carry with themselves necessary certainty, although they possess all degrees of probability, from mere probability to full but not necessary certainty.

Antecedent-probability arguments sometimes produce full certainty. If the cause certainly exists and no hinderance can arise, the effect is certain, and the proof is decisive of belief. Just so far as doubts may arise in regard to the sufficiency of the cause or the opportunity of its operating, just so far will the reasoning from this class of proofs be invalidated.

Signs possess full certainty, or higher or lower degrees of probability, according as the cause or occasion to be proved by them is more or less necessary to their existence.

The conclusiveness of examples as proofs depends on the question whether in the particular character in which they are presented as proofs, they are included in the same general law, or are determined by the same cause which is supposed in the thing to be proved. From observing the organic structure in one plant, the naturalist will safely conclude in regard to any other plant of the same species. He cannot, however, so conclude in regard to the color. But one cause can be supposed to operate in the former case; in the latter, various causes may have influence.

§ 157. From the diverse nature of the different kinds of arguments enumerated it will appear at once

that while some are applicable to all subjects, others are adapted only to particular kinds of subjects.

Analytical proofs are applicable to every kind of subject, as is obvious from their nature.

Of Synthetical proofs, the intuitive class belong distinctively to mathematical reasoning or pure science. They are employed, however, in all kinds of dis

course.

Empirical proofs are employed in all reasoning that respects matters of experience, whether the reasoning terminates on facts or on general truths.