ture: The production of motion by impulfe and gravity is an univerfal law, which has hitherto admitted of no exception. But there are other caufes, which have been found more irregular and uncertain; nor has rhubarb proved always a purge, or opium a foporific to every one, who has taken these medicines. It is true, when any caufe fails of producing its ufual effect, philofophers ascribe not this to any irregularity in nature; but suppofe, that fome fecret caufes, in the particular structure of parts, have prevented the operation. Our reafonings, however, and conclufions concerning the event are the fame as if this principle had no place. Being determined by custom to transfer the paft to the future, in all our inferences; where the past has been entirely regular and uniform, we expect the event with the greateft affurance, and leave no room for any contrary fuppofition. But where different effects have been found to follow from causes, which are to appearance exactly fimilar, all these various effects must occur to the mind in transferring the paft to the future, and enter into our confideration, when we determine the probability of the event. Though we give the preference to that which has been found most ufual, and believe that this effect will exift, we must not overlook the other effects, but muft affign to each of them a particular weight and authority, in proportion as we have found it to be more or lefs frequent. It is more probable, in almost every country of EUROPE, that there will be froft fometime in JANUARY, than that the weather will continue open throughout that whole month; though this probability varies according to the different climates, and approaches to a certainty in the more northern kingdoms. Here then it feems evident, that, when we transfer the paft to the future, in order to determine the effect, which will refult from any caufe, we transfer all the different events, in the fame propor tion as they have appeared in the paft, and conceive one to have exifted a hundred times, for inftance, another ten times, and another once. As a great number of views do here concur in one event, they fortify and confirm it to the imagination, beget that fentiment which we call belief, and give its object the preference above the contrary event, which is not supported by an equal number of experiments, and recurs not fo frequently to the thought in transferring the paft to the future. Let any one try to account for this operation of the mind upon any of the received fyftems of philofophy, and he will be fenfible of the difficulty. For my part, I fhall think it sufficient, if the prefent hints excite the curiosity of philofophers, and make them fenfible how defective all common theories are, in treating of fuch curious and fuch fublime fubjects. SECTION VII. Of the IDEA of NECESSARY CONNEXION. PART I. THE the former, being fenfible, are always clear and determinate, the fmalleft diftinction between them is immediately perceptible, and the fame terms are ftill expreffive of the fame ideas, without ambiguity or variation. An oval is never mistaken for a circle, nor an hyperbola for an elipfis. The ifoceles and scalenum are diftinguished by boundaries more exact than vice and virtue, right and wrong. If any term be defined in geometry, the mind readily, of itself, fubftitutes, on all occafions, the definition for the term defined: Or even when no definition is employed, the object itself may be prefented to the fenfes, and by that means be fteadily and clearly apprehended. But the finer fentiments of the mind, the operations of the understanding, the various agitations of the paffions, though really in themfelves diftinct, eafily escape us, when furveyed by reflection; nor is it in our power to recall the original object, as often as we have occafion to contemplate it. Ambiguity, by this means, is gradually introduced into our reasonings: Similar objects are readily taken to be the fame: And the conclufion becomes at laft very wide of the premises. One 1 One may fafely, however, affirm, that, if we confider these sciences in a proper light, their advantages and difadvantages nearly compenfate each other, and reduce both of them to a ftate of equality. If the mind, with greater facility, retains the ideas of geometry clear and determinate, it must carry on a much longer and more intricate chain of reasoning, and compare ideas much wider of each other, in order to reach the abftrufer truths of that science. And if moral ideas are apt, without extreme care, to fall into obfcurity and confufion, the inferences are always much fhorter in thefe difquifitions, and the intermediate fleps, which lead to the conclufion, much fewer than in the fciences which treat of quantity and number. In reality, there is fcarcely a propofition in EUCLID fo fimple, as not to confist of more parts, than are to be found in any moral reasoning which runs not into chimera and conceit. Where we trace the principles of the human mind through a few fteps, we may be very well fatisfied with our progress; confidering how foon nature throws a bar to all our enquiries concerning causes, and reduces us to an acknowledgment of our ignorance. The chief obftacle, therefore, to our improvement in the moral or metaphysical fciences is the obfcurity of the ideas, and ambiguity of the terms. The principal difficulty in the mathematics is the length of inferences and compass of thought, requifite to the forming any conclufion. And perhaps, our progrefs in natural philofophy is chiefly retarded by the want of proper experiments and phænomena, which are often difcovered by chance, and cannot always be found, when requifite, even by the moft diligent and prudent enquiry. As moral philofophy feems hitherto to have received lefs improvement than either geometry or phyfics, we may conclude, that, if there be any differ ence ence in this respect among these sciences, the difficulties, which obftruct the progrefs of the former, require fuperior care and capacity to be furmounted. There are no ideas, which occur in metaphyfics, more obfcure and uncertain, than thofe of power, force, energy, or neceffary connexion, of which it is every moment necessary for us to treat in all our difquifitions. Wę fhall, therefore, endeavour, in this fection, to fix, if posfible, the precife meaning of thefe terms, and thereby remove fome part of that obfcurity, which is fo much. complained of in this fpecies of philofophy. It seems a propofition, which will not admit of much difpute, that all our ideas are nothing but copies of our impreffions, or, in other words, that it is impoffible for us to think of any thing, which we have not antecedently felt, either by our external or internal fenfes. I have endeavoured to explain and prove this propofition, and have expreffed my hopes, that, by a proper application of it, men may reach a greater clearness and precifion in philofophical reafonings, than what they have hitherto been able to attain. Complex ideas may, perhaps, be well known by definition, which is nothing but an enumeration of thofe parts or fimple ideas, that compofe them. But when we have pushed up definitions to the moft fimple ideas, and find ftill fome ambiguity and obfcurity; what refource are we then poffeffed of? By what invention can we throw light upon thefe ideas, and render them altogether precife and determinate to our intellectual view? Produce the impreffions or original fentiments, from which the ideas are copied. These impreffions are all ftrong and fenfible. They admit not pf ambiguity. They are not only placed in a full light ? Section II. themselves, |