But to return to the proposed distribution of moral evidence. Under it I include these three tribes, experience, analogy, and testimony. To these I shall subjoin the consideration of a fourth, totally distinct from them all, but which appears to be a mixture of the demonstrative and the moral; or rather a particular application of the former, for ascertaining the precise force of the latter. The evidence I mean is that resulting from calculations concerning chances.
The first of these I have named peculiarly the evidence of experience, not with philosophical propriety, but in compliance with common language, and for distinction's sake. Analogical reasoning is surely reasoning from a more indirect experience. Now, as to this first kind, our experience is either uniform or various. In the one case, provided the facts on which it is founded be sufficiently numerous, the conclusion is said to be morally certain. In the other, the conclusion, built on the greater number of instances, is said to be probable, and more or less so, according to the proportion which the instances on that side bear to those on the opposite. Thus we are perfectly assured that iron thrown into the river will sink, that deal will float; because these conclusions are built on a full and uniform experience. That in the last week of December next, it will snow in any part of Britain specified, is perhaps probable; that is, if, on inquiry or recollection, we are satisfied that this hath more frequently happened than the contrary; that some time in that month it will snow, is more probable, but not certain, because, though this conclusion is founded on experience, that experience is not uniform; lastly, that it will snow some time during winter will, I believe, on the same principles, be pronounced certain.
It was affirmed that experience, or the tendency of the mind to associate ideas under the notion of causes, effects, or adjuncts, is never contracted by one example only. This assertion, it may be thought, is contradicted by the principle on which physiologists commonly proceed, who consider one accurate experiment in support of a particular doctrine as sufficient evidence. The better to explain this phenomenon, and the further to illustrate the nature of experience, I shall make the following observations. First, whereas sense and memory are conversant only about individuals, our earliest experiences imply, or perhaps generate, the notion of a species, including all those individuals which have the most obvious and universal resemblance. From Charles, Thomas, William, we ascend to the idea of man; from Britain, France, Spain, to the idea of kingdom. As our acquaintance with nature enlarges, we discover resemblances, of a striking and important nature, between one species and another, which naturally begets the notion of a genus. From comparing men with beasts, birds, fishes, and reptiles, we perceive that they are all alike possessed of life, or a principle of sensation and action, and of an organized body, and hence acquire the idea of animal: in like manner, from comparing kingdoms with republics and aristocracies, we obtain the idea of nation, and thence again rise in the same track to ideas still more comprehensive. Further, let it be remembered, that by experience we not only decide concerning the future from the past, but concerning things uncommon from things familiar which resemble them.
Now, to apply this observation: a botanist, in traversing the fields, lights on a particular plant, which appears to be of a species he is not acquainted with. The flower, he observes, is monopetalous, and the number of flowers it carries is seven. Here are two facts that occur to his observation; let us consider in what way he will be disposed to argue from them. From the first he does not hesitate to conclude, not only as probable, but as certain, that this individual, and all of the same species, invariably produce monopetalous flowers. From the second, he by no means concludes, as either certain, or even probable, that the flowers which either this plant, or others of the same species, carry at once, will always be seven. This difference, to a superficial inquirer, might seem capricious, since there appears to be one example, and but one in either case, on which the conclusion can be founded. The truth is, that it is not from this example only that he deduces these inferences. Had he never heretofore taken the smallest notice of any plant, he could not have reasoned at all from these remarks. The mind recurs instantly from the unknown to all the other known species of the same genus, and thence to all the known genera of the same order or tribe; and having experienced in the one instance, a regularity in every species, genus, and tribe, which admits no exception; in the other a variety as boundless as that of season, soil, and culture, it learns hence to mark the difference.
Again, we may observe that, on a closer acquaintance with those objects wherewith we are surrounded, we come to discover that they are mostly of a compound nature, and that not only as containing a complication of those qualities called accidents, as gravity, mobility, colour, extension, figure, solidity,which are common almost to all matter, not only as consisting of different members,but as comprehending a mixture of bodies, often very different in their nature and properties, as air, fire, water, earth, salt, oil, spirit, and the like. These, perhaps, on deeper researches, will be found to consist of materials still simpler. Moreover, as we advance in the study of nature, we daily find more reason to be convinced of her constancy in all her operations, that like causes, in like circumstances, always produce like effects, and inversely, like effects always flow from like causes. The inconstancy which appears at first in some of nature's works, a more improved experience teacheth us to account for in this manner. As most of the objects we know are of a complex nature, on a narrower scrutiny we find,that the effects ascribed to them ought often solely to be ascribed to one or more of the component parts; that the others noway contribute to the production: that, on the contrary, they sometimes tend to hinder it. If the parts in the composition of similar objects were always in equal quantity, their being compounded would make no odds; if the parts, though not equal, bore always the same proportion to the whole, this would make a difference: but such as in many cases might be computed. In both respects, however, there is an immense variety. Perhaps every individual differs from every other individual of the same species, both in the quantities and in the proportions of its constituent members and component parts. This diversity is also found in other things, which, though hardly reducible to species, are generally known by the same name. The atmosphere in the same place at different times, or at the same time in different places, differs in density, heat, humidity, and the number, quality, and proportion of the vapours or particles with which it is loaden. The more then we become acquainted with elementary natures, the more we are ascertained by a general experience of the uniformity of their operations. And though perhaps it be impossible for us to attain the knowledge of the simplest elements of any body, yet when any thing appears so simple, or rather so exactly uniform, as that we have observed it invariably to produce similar effects; on discovering any new effects, though but by one experiment, we conclude, from the general experience of the efficient, a like constancy in this energy as in the rest. Fire consumes wood, melts copper, and hardens clay. In these instances it acts uniformly, but not in these only. I have always experienced hitherto, that whatever of any species is consumed by it once, all of the same species it will consume upon trial at any time. The like may be said of what is melted, or hardened, or otherwise altered by it. If then, for the first time, I try the influence of fire on any fossil, or other substance, whatever be the effect, I readily conclude that fire will always produce a similar effect on similar bodies. This conclusion is not founded on this single instance, but on this instance compared with a general experience of the regularity of this element in all its operations.
So much for the first tribe, the evidence of experience, on which I have enlarged the more, as it is, if not the foundation, at least the criterion of all moral reasoning whatever. It is, besides, the principal organ of truth in all the branches of physiology (I use the word in its largest acceptation), including natural history, astronomy, geography, mechanics, optics, hydrostatics, meteorology, medicine, chemistry. Under the general term I also comprehend natural theology and psychology, which, in my opinion, have been most unnaturally disjoined by philosophers. Spirit, which here comprises only the Supreme Being and the human soul, is surely as much included under the notion of natural object as body is, and is knowable to the philosopher purely in the same way, by observation and experience.
The evidence of analogy, as was hinted above, is but a more indirect experience, founded on some remote similitude. As things, however, are often more easily comprehended by the aid of example than by definition, I shall in that manner illustrate the difference between experimental evidence and analogical. The circulation of the blood in one human body is, I shall suppose, experimentally discovered. Nobody will doubt of this being a sufficient proof from experience, that the blood circulates in every human body. Nay, further, when we consider the great similarity which other animal bodies bear to the human body, and that both in the structure and in the destination of the several organs and limbs; particularly when we consider the resemblance in the blood itself, and blood-vessels and in the fabric and pulsation of the heart and arteries, it will appear sufficient experimental evidence of the circulation of the blood in brutes, especially in quadrupeds. Yet, in this application, it is manifest, that the evidence is weaker than in the former. But should I from the same experiment infer the circulation of the sap in vegetables, this would be called an argument only from analogy. Now, all reasonings from experience are obviously weakened in proportion to the remoteness of the resemblance subsisting between that on which the argument is founded, and that concerning which we form the conclusion.
The same thing may be considered in a different way. I have learnt from experience, that like effects sometimes proceed from objects which faintly resemble, but not near so frequently as from objects which have a more perfect likeness By this experience I have been enabled to determine the degrees of probability from the degrees of similarity in the different cases. It is presumable that the former of these ways has the earliest influence, when the mind, unaccustomed to reflection, forms but a weak association, and consequently but a weak expectation of a similar event from a weak resemblance. The latter seems more the result of thought, and is better adapted to the ordinary forms of reasoning.
It is allowed that an analogical evidence is at best but a feeble support, and is hardly ever honoured with the name of proof. Nevertheless, when the analogies are numerous, and the subject admits not evidence of another kind, it doth notwant efficacy. It must be owned, however, that it is generally more successful in silencing objections than in evincing truth, and on this account may more properly be styled the defensive arms of the orator than the offensive. Though it rarely refutes, it frequently repels refutation, like those weapons which, though they cannot kill the enemy, will ward his blows.60
The third tribe is the evidence of testimony, which is either oral
or written. This also hath been thought by some, but unjustly, to be
solely and originally derived from the same source,
experience.61 The utmost in regard to this, that
can be affirmed with truth, is that the evidence of testimony is to
be considered as strictly logical, no further than human veracity in
general, or the veracity of witnesses of such a character, and in
such circumstances in particular, is supported, or perhaps more
properly, hath not been refuted, by experience. But that testimony,
antecedently to experience, hath a natural influence on belief, is
undeniable. In this it resembles memory; for though the defects and
misrepresentations of memory are corrected by experience, yet that
this faculty hath an innate evidence of its own we know from this,
that if we had not previously given an implicit faith to memory, we
had never been able to acquire experience. This will appear from the
revisal of its nature, as explained above. Nay, it must be owned,
that in what regards single facts, testimony is more adequate
evidence than any conclusions from experience. The immediate
conclusions from experience are general, and run thus: "This is the
ordinary course of nature;"-"Such an event may reasonably be
expected, when all the attendant circumstances are similar." When we
descend to particulars, the conclusion necessarily becomes weaker,
being more indirect. For though all the known circumstances be
similar; all the actual circumstances may not be similar; nor
is it possible in any case to be assured, that all the actual
circumstances are known to us. Accordingly, experience is the
foundation of philosophy; which consists in a collection of general
truths, systematically digested. On the contrary, the direct
conclusion from testimony is particular, and runs thus: "This is the
fact in the instance specified." Testimony, therefore, is the
foundation of
history, which is occupied about individuals . Hence we derive our acquaintance with past ages, as from experience we derive all that we can discover of the future. But the former is dignified with the name of knowledge, whereas the latter is regarded as matter of conjecture only. When experience is applied to the discovery of the truth in a particular incident, we call the evidence presumptive; ample testimony is accounted a positive proof of the fact. Nay, the strongest conviction built merely on the former is sometimes overturned by the slightest attack of the latter. Testimony is capable of giving us absolute certainty (Mr. Hume himself being judge62) even of the most miraculous fact, or of what is contrary to uniform experience. For, perhaps, in no other instance can experience be applied to individual events with so much certainty, as in what relates to the revolutions of the heavenly bodies. Yet, even this evidence, he admits, may not only be counterbalanced, but destroyed by testimony.
But to return. Testimony is a serious intimation from another, of any fact or observation, as being what he remembers to have seen or heard or experienced. To this, when we have no positive reasons of mistrust or doubt, we are, by an original principle of our nature (analogous to that which compels our faith in memory), led to give an unlimited assent. As on memory alone is founded the merely personal experience of the individual, so on testimony in concurrence with memory is founded the much more extensive experience which is not originally our own, but derived from others.63 By the first, I question not, a man might acquire all the knowledge necessary for mere animal support, in that rudest state of human nature (if ever such a state existed) which was without speech and without society; to the last, in conjunction with the other, we are indebted for every thing which distinguishes the man from the brute, for language, arts, and civilization. It hath been observed, that from experience we learn to confine our belief in human testimony within the proper bounds. Hence we are taught to consider many attendant circumstances, which serve either to corroborate or to invalidate its evidence. The reputation of the attester, his manner of address, the nature of the fact attested, the occasion of giving the testimony, the possible or probable design in giving it, the disposition of the hearers to whom it was given, and several other circumstances, have all considerable influence in fixing the degree of credibility. But of these I shall have occasion to take notice afterwards. It deserves likewise to be attended to on this subject, that in a number of concurrent testimonies (in cases wherein there could have been no previous concert), there is a probability distinct from that which may be termed the sum of the probabilities resulting from the testimonies of the witnesses, a probability which would remain even though the witnesses were of such a character as to merit no faith at all. This probability arises purely from the concurrence itself. That such a concurrence should spring from chance is as one to infinite; that is, in other words, morally impossible. If therefore concert be excluded, there remains no other cause but the reality of the fact.
Now to this species of evidence, testimony, we are first immediately indebted for all the branches of philology, such as, history, civil, ecclesiastic, and literary; grammar, languages, jurisprudence, and criticism; to which I may add revealed religion, as far as it is to be considered as a subject of historical and critical inquiry, and so discoverable by natural means: and secondly, to the same source we owe, as was hinted above, a great part of that light which is commonly known under the name of experience, but which is, in fact, not founded on our own personal observations, or the notices originally given by our own senses, but on the attested experiences and observations of others. So that as hence we derive entirely our knowledge of the actions and productions of men, especially in other regions and in former ages, hence also we derive, in a much greater measure than is commonly imagined, our acquaintance with Nature and her works-Logic, rhetoric, ethics, economics, and politics are properly branches of pneumatology, though very closely connected with the philological studies above enumerated.
The last kind of evidence I proposed to consider, was that resulting from calculations of chances. Chance is not commonly understood, either in philosophic or in vulgar language, to imply the exclusion of a cause, but our ignorance of the cause. It is often employed to denote a bare possibility of an event, when nothing is known either to produce or to hinder it. But in this meaning it can never be made the subject of calculation. It then only affords scope to the calculator, when a cause is known for the production of an effect, and when that effect must necessarily be attended with this or that or the other circumstance; but no cause is known to determine us to regard one particular circumstance in preference to the rest, as that which shall accompany the supposed effect. The effect is then considered as necessary, but the circumstance as only casual or contingent. When a die is thrown out of the hand, we know that its gravity will make it fall; we know also that this, together with its cubical figure, will make it lie so, when intercepted by the table, as to have one side facing upwards. Thus far we proceed on the certain principles of a uniform experience; butthere is no principle which can lead me to conclude that one side rather than another will be turned up. I know that this circumstance is not without a cause; but is, on the contrary, as really effected by the previous tossing which it receives in the hand or in the box, as its fall and the manner of its lying are by its gravity and figure. But the various turns or motions given it, in this manner, do inevitably escape my notice; and so are held for nothing. I say, therefore, that the chance is equal for every one of the six sides. Now, if five of these were marked with the same figure, suppose a dagger [†], and only one with an asterisk [*], I should in that case say, there were five chances that the die would turn up the dagger, for one that it would turn up the asterisk. For the turning up each of the six sides being equally possible, there are five cases in which the dagger, and only one in which the asterisk would be uppermost.
This differs from experience, inasmuch as I reckon the probability here, not from numbering and comparing the events after repeated trials, but without any trial, from balancing the possibilities on both sides. But though different from experience, it is so similar, that we cannot wonder that it should produce a similar effect upon the mind. These different positions being considered as equal, if any of five shall produce a similar effect, and but the sixth another, the mind, weighing the different events, resteth in an expectation of that in which the greater number of chances concur; but still accompanied with a degree of hesitancy, which appears proportioned to the number of chances on the opposite side. It is much after the same manner that the mind, on comparing its own experiences, when five instances favour one side to one that favours the contrary, determines the greater credibility of the former. Hence, in all complicated cases, the very degree of probability may be arithmetically ascertained. That two dice marked in the common way will turn up seven, is thrice as probable as that they will turn up eleven, and six times as probable as that they will turn up twelve.64 The degree of probability is here determined demonstratively. It is indeed true that such mathematical calculations may be founded on experience, as well as upon chances. Examples of this we have in the computations that have been made of the value of annuities, insurances, and several other commercial articles. In such cases a great number of instancesis necessary, the greatest exactness in collecting them on each side, and due care that there be no discoverable peculiarity in any of them, which would render them unfit for supporting a general conclusion.
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