ALL the objects of human reason or enquiry may naturally be divided into two kinds, viz. Re|lations of Ideas and Matters of Fact. Of the first kind are the sciences of Geometry, Algebra, and Arith|metic; and in short, every affirmation, which is either intuitively or demonstratively certain. That the square of the hypothenuse is equal to the squares of the two sides, is a proposition, which expresses a relation between these figures. That three times five is equal to the half of thirty, expresses a relation between these numbers. Propositions of this kind are discoverable by the mere operation of thought, without dependence on what is any where existent in the universe. Tho' there never were a circle or triangle in nature, the truths demon|strated by EUCLID, would for ever retain their cer|tainty and evidence.
Page 46 MATTERS of fact, which are the second objects of human reason, are not ascertained int the same man|ner; nor is our evidence of their truth, however great, of a like nature with the foregoing. The con|trary of every matter of fact is still possible; be|cause it can never imply a contradiction, and is con|ceived by the mind with equal facility and distinct|ness, as if ever so conformable to reality. That the sun will not rise to-morrow is no less intelligible a proposition, and implies no more contradiction, than the affirmation, that it will rise. We should in vain, therefore, attempt to demonstrate its falshood. Were it demonstratively false, it would imply a contradic|tion, and could never be distinctly conceived by the mind.
IT may, therefore, be a subject worthy curiosity, to enquire what is the nature of that evidence, which assures us of any real existence and matter of fact, beyond the present testimony of our senses, or the re|cords of our memory. This part of philosophy, 'tis observable, has been little cultivated, either by the ancients or moderns; and therefore our doubts and er|rors, in the prosecution of so important an enquiry, may be the more excusable, while we march thro' such difficult paths, without any guide or direction. They may even prove useful, by exciting curiosity, and destroying that implicit faith and security, which is the bane of all reasoning and free enquiry. The Page 47 discovery of defects in the common philosophy, if any such there be, will not, I presume, be a discou|ragement, but rather an incitement, as is usual, to attempt something more full and satisfactory, than has yet been proposed to the public.
ALL reasonings concerning matter of fact seem to be founded in the relation of Cause and Effect. By means of that relation alone can we go beyond the evidence of our memory and senses. If you were to ask a man, why he believes any matter of fact, which is absent; for instance, that his friend is in the coun|try, or in FRANCE; he would give you a reason; and this reason would be some other fact; as a letter received from him, or the knowlege of his former resolutions and primises. A man, finding a watch or any other machine in a desart island, would con|clude, that there had once been men in that island. All our reasonings concerning fact are of the same nature. And here 'tis constantly supposed, that there is a connexion between the present fact and that in|ferred from it. Were there nothing to bind them to|gether, the inference would be entirely precarious. The hearing of an articulate voice and rational dis|course in the dark assures us of the presence of some person: Why? because these are the effects of the human make and fabric, and closely connected with it. If we anatomize all the other reasonings of this nature, we shall find, that they are founded in the Page 48 relation of cause and effect, and that this relation is either near or remote, direct or collateral. Heat and light are collateral effects of fire, and the one effect may justly be inferred from the other.
IF we would satisfy ourselves, therefore, concern|ing the nature of that evidence, which assures us of all matters of fact, we must enquire how we arrive at the knowlege of cause and effect.
I SHALL venture to affirm, as a general proposition, which admits of no exception, that the knowlege of this relation is not, in any instance, attained by reasonings à priori; but arises entirely from experience, when we find, that any particular objects are constantly con|joined with each other. Let an object be presented to a man of ever so strong natural reason and abilities; if that object be entirely new to him, he will not be able, by the most accurate examination of its sensible qualities, to discover any of its causes or effects. ADAM, tho' his rational faculties be supposed, at the very first, entirely perfect, could not have inferred from the fluidity and transparency of water, that it would suffocate him, or from the light and warmth of fire, that it would consume him. No object ever dis|covers, by the qualities which appear to the senses, either the causes, which produced it, or the effects, which will arise from it; nor can our reason, unas|sisted Page 49 by experience, ever draw any inferences con|cerning real existence and matter of fact.
THIS proposition, that cause and effects are discover|able, not by reason but by experience, will readily be admitted with regard to such objects, as we remem|ber to have been once altogether unknown to us; since we must be conscious of the utter inability which we then lay under of foretelling what would arise from them. Present two smooth pieces of marble to a man, who has no tincture of natural philosophy; he will never discover, that they will adhere together, in such a manner as to require great force to separate them in a direct line, while they make so small a re|sistance to a lateral pressure. Such events, as bear little analogy to the common course of nature, are also readily confessed to be known only by experience; nor does any man imagine that the explosion of gunpowder, or the attraction of a loadstone could ever be disco|vered by arguments à priori. In like manner, when an effect is supposed to depend upon an intricate ma|chinery or secret structure of parts, we make no dif|ficulty to attribute all our knowlege of it to experi|ence. Who will assert, that he can give the ultimate reason, why milk or bread is proper nourishment for a man, not for a lion or a tyger?
BUT the same truth may not appear, at first sight, to have the same evidence with regard to events, Page 50 which have become familiar to us from our first ap|pearance in the world, which bear a close analogy to the whole course of nature, and which are supposed to depend on the simple qualities of objects, without any secret structure of parts. We are apt to ima|gine, that we could discover these effects, by the mere operations of our reason, without experience. We fancy, that, were we brought, on a sudden, into this world, we could at first have inferred, that one Bil|liard-ball would communicate motion to another up|on impulse; and that we needed not to have waited for the event, in order to pronounce with certainty concerning it. Such is the influence of custom, that, where it is strongest, it not only covers our natural ignorance, but even conceals itself, and seems not to take place, merely because it is found in the highest degree.
BUT to convince us, that all the laws of nature and all the operations of bodies, without exception, are known only by experience, the following reflec|tions, may, perhaps, suffice. Were any object pre|sented to us, and were we required to pronounce con|cerning the effect, which will result from it, without consulting past observation; after what manner, I be|seech you, must the mind proceed in this operation? It must invent or imagine some event, which it ascribes to the object as its effect; and 'tis plain that this invention must be entirely arbitrary. The mind can Page 51 never possibly find the effect in the supposed cause, by the most accurate scrutiny and examination. For the effect is totally different from the cause, and conse|quently can never be discovered in it. Motion in the second Billiard-ball is a quite distinct event from mo|tion in the first; nor is there any thing in the one to suggest the smallest hint of the other. A stone or piece of metal raised into the air, and left without any support, immediately falls: But to consider the matter à priori, is there any thing we discover in this situation, which can beget the idea of a downward, rather than an upward, or any other motion, in the stone or metal?
AND as the first imagination or invention of a par|ticular effect, in all natural operations, is arbitrary, where we consult not experience; so must we also esteem the supposed tye or connexion between the cause and effect, which binds them together, and ren|ders it impossible, that any other effect could result from the operatioon of that cause. When I see, for instance, a Billiard-ball moving in a strait line towards another; even suppose motion in the second ball should by accident be suggested to me, as the result of their contact or impulse; may I not conceive, that a hundred different events might as well follow from that cause? May not both these balls remain at abso|lute rest? May not the first ball return in a strait line, or leap off from the second in any line or direction? Page 52 All these suppositions are consistent and conceivable. Why then should we give the preference to one, which is no more consistent nor conceivable than the rest? All our reasonings à priori will never be able to shew us any foundation for this preference.
IN a word, then, every effect is a distinct event from its cause. It could not, therefore, be discovered In the cause, and the first invention or conception of it, à priori, must be entirely arbitrary. And even after it is suggested, the conjunction of it with the cause must appear equally arbitrary; since there are always many other effects, which, to reason, must seem fully as consistent and natural. In vain, there|fore, should we pretend to determine any single event, or infer any cause or effect, without the assistance of observation and experience.
HENCE we may discover the reason, why no philo|sopher, who is rational and modest, has ever pretended to assign the ultimate cause of any natural operation, or to show distinctly the action of that power, which produced any single effect in the universe. 'Tis con|fessed, that the utmost effort of human reason is, to reduce the principles, productive of natural phaeno|mena, to a greater simplicity, and to resolve the many particular effects into a few general cause, by means of reasonings from analogy, experience, and observation. But as to the causes of these general Page 53 causes, we should in vain attempt their discovery; nor shall we ever be able to satisfy ourselves, by any particular explication of them. These ultimate springs and principles are totally shut up from human curi|osity and enquiry. Elasticity, gravity, cohesion of parts, communication of motion by impulse; these are probably the ultimate causes and principles which we shall ever discover in nature; and we may esteem ourselves sufficiently happy, if, by accurate enquiry and reasoning, we can trace up the particular phaeno|mena to, or near to, these general principles. The most perfect philosophy of the natural kind only staves off our ignorance a little longer: As perhaps the most perfect philosophy of the moral or metaphy|sical kind serves only to discover larger portion of our ignorance. Thus the observation of human blindness and weakness is the result of all philosophy, and meets us, at every turn, in spight of our endea|vours to elude or avoid it.
NOR is geometry, when taken into the assistance of natural philosophy, ever able to remedy this de|fect, or lead us into the knowlege of ultimate causes, by all that accuracy of reasoning, for which it is so justly celebrated. Every part of mixed mathematics goes upon the supposition, that certain laws are esta|blished by nature in her operations; and abstract rea|sonings are employed, either to assist experience in the discovery of these laws, or to determine their in|fluence Page 54 in particular instances, where it depends upon any precise degrees of distance and quantity. Thus 'tis a law of motion, discovered by experience, that the moment or force of any body in motion is in the compound ratio or proportion of its solid contents and its velocity; and consequently, that a small sorce may remove the greatest obstacle or raise the greatest weight, if by any contrivance or machinery we can encrease the velocity of that force, so as to make it an over|match for its antagonist. Geometry assists us in the application of this law, by giving us the just di|mensions of all the parts and figures, which can enter into any species of machine; but still the discovery of the law itself is owing merely to experience, and all the abstract reasonings in the world could never lead us one step towards the knowlege of it. When we reason à priori, and consider merely any object or cause, as it appears to the mind, independent of all observation, it never could suggest to us the notion of any distinct object, such as its effect; much less, shew us the inseparable and inviolable connection be|tween them. A man must be very sagacious, who could discover by reasoning, that crystal is the effect of heat, and ice of cold, without being previously acquainted with the operations of these qualities.