sceptical doubts concerning the operations of the understanding part i all the objects of human reason or enquiry may naturally be divided into two kinds to wit relations of ideas and matters of fact of the first kind are the sciences of geometry algebra and arithmetic and in short every affirmation which is either intuitively or demonstratively certain that the square of the hypothenuse is equal to the square 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 anywhere existent in the universe though there never were a circle or triangle in nature the truths demonstrated by euclid would for ever retain their certainty and evidence matters of fact which are the second objects of human reason are not ascertained in the same manner nor is our evidence of their truth however great of a like nature with the foregoing the contrary of every matter of fact is still possible because it can never imply a contradiction and is conceived by the mind with the same facility and distinctness 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 falsehood were it demonstratively false it would imply a contradiction and could never be distinctly conceived by the mind it may therefore be a subject worthy of 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 records of our memory this part of philosophy it is observable has been little cultivated either by the ancients or moderns and therefore our doubts and errors in the prosecution of so important an enquiry may be the more excusable while we march through 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 discovery of defects in the common philosophy if any such there be will not i presume be a discouragement 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 on the relation of cause and effect by means of that relation alone we can 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 country 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 knowledge of his former resolutions and promises a man finding a watch or any other machine in a desert island would conclude that there had once been men in that island all our reasonings concerning fact are of the same nature and here it is constantly supposed that there is a connexion between the present fact and that which is inferred from it were there nothing to bind them together the inference would be entirely precarious the hearing of an articulate voice and rational discourse 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 on the 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 concerning the nature of that evidence which assures us of matters of fact we must enquire how we arrive at the knowledge of cause and effect i shall venture to affirm as a general proposition which admits of no exception that the knowledge of this relation is not in any instance attained by reasonings a priori but arises entirely from experience when we find that any particular objects are constantly conjoined 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 though 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 discovers 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 unassisted by experience ever draw any inference concerning real existence and matter of fact this proposition that causes and effects are discoverable not by reason but by experience will readily be admitted with regard to such objects as we remember to have once been 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 resistance 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 discovered by arguments a priori in like manner when an effect is supposed to depend upon an intricate machinery or secret structure of parts we make no difficulty in attributing all our knowledge of it to experience 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 tiger but the same truth may not appear at first sight to have the same evidence with regard to events which have become familiar to us from our first appearance 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 imagine that we could discover these effects by the mere operation 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 billiard ball would communicate motion to another upon 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 reflections may perhaps suffice were any object presented to us and were we required to pronounce concerning the effect which will result from it without consulting past observation after what manner i beseech 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 it is plain that this invention must be entirely arbitrary the mind can 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 consequently can never be discovered in it motion in the second billiard ball is a quite distinct event from motion in the first nor is there anything 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 a priori is there anything 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 particular effect in all natural operations is arbitrary where we consult not experience so must we also esteem the supposed tie or connexion between the cause and effect which binds them together and renders it impossible that any other effect could result from the operation of that cause when i see for instance a billiard ball moving in a straight 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 absolute rest may not the first ball return in a straight line or leap off from the second in any line or direction all these suppositions are consistent and conceivable why then should we give the preference to one which is no more consistent or conceivable than the rest all our reasonings a priori will never be able to show 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 a 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 therefore 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 philosopher 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 produces any single effect in the universe it is confessed that the utmost effort of human reason is to reduce the principles productive of natural phenomena to a greater simplicity and to resolve the many particular effects into a few general causes by means of reasonings from analogy experience and observation but as to the causes of these general 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 curiosity 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 phenomena 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 metaphysical kind serves only to discover larger portions of it thus the observation of human blindness and weakness is the result of all philosophy and meets us at every turn in spite of our endeavours to elude or avoid it nor is geometry when taken into the assistance of natural philosophy ever able to remedy this defect or lead us into the knowledge of ultimate causes by all that accuracy of reasoning for which it is so justly celebrated every part of mixed mathematics proceeds upon the supposition that certain laws are established by nature in her operations and abstract reasonings are employed either to assist experience in the discovery of these laws or to determine their influence in particular instances where it depends upon any precise degree of distance and quantity thus it is 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 force may remove the greatest obstacle or raise the greatest weight if by any contrivance or machinery we can increase the velocity of that force so as to make it an overmatch for its antagonist geometry assists us in the application of this law by giving us the just dimensions 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 knowledge of it when we reason a 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 show us the inseparable and inviolable connexion between 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 operation of these qualities 