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force and vigour; renders its influence on the palfions and affections more fensible; and in a word, begets that reliance or security, which constitutes the nature of belief and opinion.
The case is the same with the probability of causes as with that of chance: There are some causes which are entirely uniform and constant in producing a particular effect; and no instance has ever yet been found of any failure or irregularity in their operation. Fire has always burned, and water suffocated; every human creature: The production of motion by impulse and gravity is an universal law, which has hitherto admitted of no exception. But there are other causes, which have been found more irregular and uncertain ; nor has rhubarb always proved a purge, or opium a soporific, to every one who has taken these medicines. It is true, when any cause fails of producing its usual effect; philofo. phers ascribe not this to any irregularity in nature ; but suppose, that some secret causes, in the particular structure of parts, have prevented the operation. Our reasonings, however, and conclusions concerning the event, are the same as if this principle had no place. Being determined by custom to transfer the past to the future, in all our inferences; where the past has been entirely regular and uniform we expect the event with the greatest assurance, and leave no room for any contrary supposition. But where different effects have been found to follow from causes, which are to appearance exactly similar, all these various effects must occur to the mind in transferring the past to the future, and enter into our consideration, when we determine the probability of the event. Though we give the preference to that which has been found most usual, and believe that this effect will exist, we must not overlook the other effects, but must aflign to each of them a particular weight and authority, in proportion as we have found it to be more or less frequent. It is more proE 2
bable, in almost every country of EUROPE, that there will be frost sometime 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 seems evident, that, when we transfer the past to the future, in order to determine the effect, which will result from any cause, we transfer all the different events, in the fame proportion as they have appeared in the past, and conceive one to have exifted a hundred times, for instance, 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 sentiment 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 so frequently to the thought in transferring the past to the future. Let any one try to account for this operation of the mind upon any of the received systems of philofophy, and he will be sensible of the difficulty. For my part, I shall think it fufficient, if the present hints excite the curiosity of philosophers, and make them sensible how defective all common theories are in treating of such curious and fuch fublime subjects.
Of the IDEA of NECESSARY CONNECTION.
HE great advantage of the mathematical sciences
above the moral consists in this, that the ideas of the former, being sensible, are always clear and determinate, the smallest distinction between them is immediately perceptible, and the same terms are still expressive of the same ideas, without ambiguity or variation. An oval is never mistaken for a circle, nor an hyperbola for an ellipfis. The isosceles and scalenum are distinguished by boundaries more exact than vice and virtue, right and wrong. If any term be defined in geometry, the mind readily, of itself, substitutes, on all occasions, the definition for the term defined : Or even when no definition is employed, the object itself may be presented to the lenses, and by that means be iteadily and clearly apprehended. But the finer sentiments of the mind, the operations of the understanding, the various agitations of the passions, though really in themselves distinct, easily escape us, when surveyed by reflection; nor is it in our power to recal the original object, as often as we have occasion to contemplate it. Ambiguity, by this means, is gradually introduced into our reasonings: Similar objects are readily taken to be the same': And the conclusion becomes at last very wide of the premises. One may safely, however, affirm, that, if we con
sider these sciences in a proper light, their advantages and disadvantages nearly compensate each other, and reduce both of them to a state 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 abstruser truths of that science. And if moral ideas are apt, without extreme care, to fall into obscurity and confofion, the inferences are always much shorter in these disquisitions, and the interinediate steps, which lead to the conclusion, much fewer, than in the sciences which treat of quantity and number. In reality, there is scarcely a proposition in Euclid so simple, 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 steps, we may be very well satisfied 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 obstacle, therefore, to our improvement in the moral or metaphysical sciences, is theobscurity of the ideas, and ambiguity of the terms. The principal difficulty in the mathematics is the length of inferences and compass of thought, requilite to the forming of any conclusion. And, perhaps, our progress in natural philofophy is chiefly retarded by the want of proper experiments and phænomena, which are often discovered by chance, and cannot always be found, when requisite, even by the most diligent and prudent enquiry. As moral philofophy seems hitherto 'to have received less improvement than either geometry or phyfics, we may conclude, that if there be any difference in this respect among these sciences, the difficulties, which obstruct the progress of the former, require superior care and capacity to be furmounted.
There are no ideas, which occur in metaphysics, more obfcure and uncertain, than those of power, Force, energy, or necessary connection ; of which it is every moment necessary for us to treat in all our difquisitions. We shall, therefore, endeavour, in this -fection, to fix, if possible, the precise meaning of these terms, and thereby remove some part of that obscurity which is fo much complained of in this fpecies of philosophy,
It seems a proposition, which will not admit of much dispute, that all our ideas are nothing but copies of our impressions, or, in other words, that it is impossible 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 proposition, and have expressed my hopes, that, by a proper application of it, men may reach a greater clearness and precision in philosophical reasonings, 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 those parts or simple ideas that compofe them. But when we have pushed up definitions to the most simple ideas, and find still fome ambiguity and obscurity; what resource are we then possessed of? By what invention can we throw light upon these ideas, and render them altogether precise and determinate to our intellectual view ? Produce the impressions or original sentiments from which the ideas are copied. These impressions are all strong and sensible. They admit not of ambiguity. They are not only placed in a full light themselves, but may throw light on their correspondent ideas, which lie in obscurity. And by this means we may, perhaps, attain a new microscope or species of optics, by which, in the moral sciences, the most minute and moft simple ideas may be so enlarged as to fall seadily under our apprehension, and be equally
known * Section II.