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OF DEDUCTIVE EVIDENCE.
Part I. - Division of the subject into scientific and moral, with the
principal distinctions between them.
All rational or deductive evidence is derived from one or other of these two sources: from the invariable properties or relations of general ideas; or from the actual, though perhaps, variable connexions, subsisting among things. The former we call demonstrative, the latter moral. Demonstration is built on pure intellection, and consisteth in an uninterrupted series of axioms. That propositions formerly demonstrated are taken into the series doth not in the least invalidate this account; inasmuch as these propositions are all resolvable into axioms, and are admitted as links in the chain; not because necessary, but merely to avoid the useless prolixity which frequent and tedious repetitions of proofs formerly given would occasion. Moral evidence is founded on the principles we have from consciousness and common sense, improved by experience; and as it proceeds on this general presumption or moral axiom, that the course of nature in time to come will be similar to what it hath been hitherto, it decides, in regard to particulars, concerning the future from the past, and concerning things unknown, from things familiar to us. The first is solely conversant about number and extension, and about those other qualities which are measurable by these. Such are duration, velocity, and weight. With regard to such qualities as pleasure and pain, virtue and vice, wisdom and folly, beauty and deformity, though they admit degrees, yet, as there is no standard or common measure, by which their differences and proportions can be ascertained and expressed in numbers, they can never become the subject of demonstrative reasoning. Here rhetoric, it must be acknowledged, hath little to do. Simplicity of diction, and precision in arrangement, whence results perspicuity, are, as was observed already,* all the requisites. The proper province of rhetoric is the second, or moral evidence; for to the second belong all decisions concerning fact, and things without us.
But that the nature of moral evidence may be better understood, it will not be amiss to remark a few of the most eminent differences between this and the demonstrative.
The first difference that occurs is in their subjects. The subject of the one is, as hath been observed, abstract independent truth, or the unchangeable and necessary relations of ideas; that of the other, the real, but often changeable and contingent connexions that subsist among things actually existing. Abstract truths, as the properties of quantity, have no respect to time or to place, no dependence on the
volition of any being, or on any cause whatever, but are eternally and immutably the same. The very reverse of all this generally obtains with regard to fact. In consequence of what has been now advanced, assertions opposite to truths of the former kind are not only false, but absurd. They are not only not true, but it is impossible they should be true, whilst the meanings of the words (and consequently the ideas conipared) remain the same. This doth not hold commonly in any other kind of evidence. Take, for instance, of the first kind, the following affirmations : “ The cube of two is the half of sixteen.” - The square of the hypothenuse is equal to the sum of the squares of the sides.” If equal things be taken from equal things, the remainders will be equal.” Contrary propositions, as, “The cube of two is more than the half of sixteen." -" The square of the hypothenuse is less than the sum of the squares of the sides.” — " If equal things be taken from equal things, the remainders will be unequal,” are chargeable, not only with falsity, but with absurdity, being inconceivable and contradictory. Whereas, to these truths which we acquire by moral evidence, “ Cæsar overcame Pompey.” — “ The sun will rise to-morrow." -“ All men will die,” the opposite assertions, though untrue, are easily conceivable without changing, in the least, the import of the words, and therefore do not imply a contradiction.
The second difference I shall remark is, that moral evidence admits degrees, demonstration doth not. This is a plain consequence of the preceding difference Essential or necessary truth, the sole object of the latter, is incompatible with degree. And though actual truth, or matter of fact, be the ultimate aim of the former, likelihood alone, which is susceptible of degree, is usually the utmost attainment. Whatever is exhibited as demonstration is either mere illusion, and so no evidence at all, or absolutely perfect. There is no medium. In moral reasoning we ascend from possibility, by an insensible gradation, to probability, and thence, in the same manner, to the summit of moral certainty. On this summit, or on any of the steps leading to it, the conclusion of the argument may rest. Hence the result of that is, by way of eminence, denominated science ; and the evidence itself is termed scientific ; the result of this is frequently (not always) entitled to no higher denomination than opinion. Now, in the mathematical sciences, no mention is ever made of opinions.
The third difference is, that in the one there never can be any contrariety of proofs; in the other, there not only may, but almost always is. If one denionstration were ever capable of being refuted, it could be solely by another demonstration, this being the only sort of evidence adapted to the subject, and the only sort by which the former could be matched. But, to suppose that contraries are demonstrable, is to suppose that the same proposition is both true and false, which is a manifest contradiction. Consequently, if there should ever be the appearance of demonstration on opposite sides, that on one side must be fallacious and sophistical. It is not so with moral evidence, for unless in a few singular instances, there is always real, not apparent, evidence on both sides. There are contrary experiences, contrary presumptions, contrary testimonies, to balance
against one another. In this case, the probability, upon the whole, is in the proportion which the evidence on the side that preponderates bears to its opposite. We usually say, indeed, that the evidence lies on such a side of the question, and not on the reverse; but by this expression is only meant the overplus of evidence, on comparing both sides. In like manner, when we affirm of an event, that it is probable, we say the contrary is only possible, although, when they are severally considered, we do not scruple to say, This is more probable than that; or, The probabilities on one side outweigh those on the other.
The fourth and last difference I shall observe is, that scientific evidence is simple, consisting of only one coherent series, every part of which depends on the preceding, and, as it were, suspends the following: moral evidence is generally complicated, being in reality a bundle of independent proofs. The longest demonstration is but one uniform chain, the links whereof, taken severally, are not to be regarded as so many arguments, and consequently, when thus taken, they conclude nothing ; but taken together, and in their proper order, they form one argument, which is perfectly conclusive. It is true, the same theorem may be demonstrable in different ways, and by different mediums; but as a single demonstration clearly understood commands the fullest conviction, every other is superfluous. After one demonstrative proof, a man may try a second, purely as an exercise of ingenuity, or the better to assure himself that he hath not committed an oversight in the first. Thus it may serve to warrant the regular procedure of his faculties, but not to make an addition to the former proof, or supply any deficiency perceived in it. So far is it from answering this end, that he is no sooner sensible of a defect in an attempt of this nature, than the whole is rejected as good for nothing, and carrying with it no degree of evidence whatever. In moral reasoning, on the contrary, there is often a combination of many distinct topics of argument, no way dependent on one another. Each hath a certain portion of evidence belonging to itself, each bestows on the conclusion a particular degree of likelihood, of all which accumulated, the credibility of the fact is compounded. The former may be compared to an arch, no part of which can subsist independently of the rest. If you make any breach in it, you destroy the whole. The latter may be compared to a tower, the height whereof is but the aggregate of the heights of the several parts reared above one another, and so may be gradually diminished, as it was gradually raised.
So. much for the respective natures of scientific and of moral evidence, and those characteristical qualities which discriminate them from each other. On a survey of the whole, it seems indubitable, that if the former is infinitely superior in point of authority, the latter no less excels in point of importance. Abstract truth, as far as it is the object of our faculties, is almost entirely confined to quantity, concrete or discrete. The sphere of Demonstration is narrow, but within her sphere she is a despotic sovereign, her sway is uncontrollable. Her rival, on the contrary, hath less power, but wider empire. Her forces, indeed, are not always irresistible; but the whole world is comprised in her dominions. Reality or fact comprehends the laws and the works of nature, as well as the arts and the institutions of men; in brief, all the beings which fall under the cognizance of the human mind, with all their modifications, operations and effects. By the first, we must acknowledge, when applied to things, and combined with the discoveries of the second, our researches into nature in a certain line are facilitated, the understanding is enlightened, and many of the arts, both elegant and useful, are improved and perfected. Without the aid of the second, society must not only suffer, but perish. Human nature itself could not subsist. This organ of knowledge, which extends its influence to every precinct of philosophy, and governs in most, serves also to regulate all the ordinary, but indispensable, concernments of life. To these it is admirably adapted, notwithstanding its inferiority in respect of dig. nity, accuracy, and perspicuity. For it is principally to the acquisitions procured by experience, that we owe the use of language, and the knowledge of almost every thing that makes the soul of a man differ from that of a new-born infant. On the other hand, there is no despot so absolute, as not to be liable to a check on some side or other, and that the prerogatives of demonstration are not so very considerable, as on a cursory view one is apt to imagine; that this, as well as every other operation of the intellect, must partake in the weakness incident to all our mental faculties, and inseparable from our nature, I shall afterwards take an opportunity particularly to evince.
Part II. — The nature and origin of Experience.
I SHOULD now consider the principal tribes comprehended under the general name of moral evidence ; but, that every difficulty may be removed, which might retard our progress in the proposed discussion, it will be necessary, in the first place, to explore more accurately those sources in our nature, which give being to experience, and consequently to all those attainments, moral and intellectual, that are derived from it. These sources are two, sense and memory. The senses, both external and internal, are the original inlets of perception. They inform the mind of the facts, which in the present instant, are situated within the sphere of their activity, and no sooner discharge their office in any particular instance, than the articles of information exhibited by them, are devolved on the memory. Remembrance instantly succeeds sensation, insomuch that the memory becomes the sole repository of the knowledge received from sense; knowledge which, without this repository, would be as instantaneously lost as it is gotten, and could be of no servce to the mind. Our sensation would be no better than the fleeting pictures of a moving object on a camera obscura, which leave not the least vestige behind them. Memory, therefore, is the only original voucher extant, of those past realities for which we had once the evidence of sense. Her ideas are, as it were, the prints that have been left by sensible impressions. But from these two faculties, considered in
themselves, there results to us the knowledge only of individual facts, and only of such facts as either heretofore have come, or at present do come under the notice of our senses.
Now, in order to render this knowledge useful to us, in discovering the nature of things, and in regulating our conduct, a further process of the mind is necessary, which deserves to be carefully attended to, and may be thus illustrated. I have observed a stone fall to the ground, when nothing intervened to impede its motion. This single fact produces little or no effect on the mind beyond a bare remembrance. At another time, I observe the fall of a tile, at another of an apple, and so of almost every kind of body in the like situation. Thus my senses first, and then my memory, furnish me with numerous examples, which, though different in every other particular, are similar in this, that they present a body moving downwards, till obstructed either by the ground or by some intervenient object. Hence my first notion of gravitation. For, with regard to the similar circumstances of different facts, as by the repetition such circumstances are more deeply imprinted, the mind acquires a habit of retaining them, omitting those circumstances peculiar to each, wherein their difference consists. Hence, if objects of any kind, in a particular manner circumstanced, are remembered to have been usually, and still more if uniformly, succeeded by certain particular consequences, the idea of the former, in the supposed circumstance introduced into the mind, immediately associates the idea of the latter ; and if the object itself, so circumstanced, be presented to the senses, the mind instantly anticipates the appearance of the customary consequence. This holds also inversely. The retention and association, obove explained, are called experience. The anticipation is in effect no other than a particular conclusion from that experience. Here we may remark, by the way, that though memory gives birth to experience, which results from the comparison of facts remembered, the experience or habitual association remains, when the individual facts on which it is founded are all forgotten. I know from an experience, which excludes all doubt, the power of fire in melting silver, and yet may not be able at present to recollect a particular instance in which I have seen this effect produced, or even in which I have had the fact attested by a credible witness.
Some will perhaps object, that the account now given makes our experimental reasoning look like a sort of mechanism, necessarily resulting from the very constitution of the mind. I acknowledge the justness of the remark, but do not think that it ought to be regarded as an objection. It is plain, that our reasoning in this way, if you please to call it so, is very early, and precedes all reflection on our faculties, and the manner of applying them. Those who attended to the progress of human nature through its different stages, and through childhood in particular, will observe, that children make great acquisitions in knowledge from experience, long before they attain the use of speech. The beasts also, in their sphere, improve by experience, which hath in them just the same foundations of sense and memory as in us, and hath, besides, a similar influence on their actions. it is precisely in the same manner, and with the same success, that you