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laws of gravitation had, at the moment at which the rebound begins, been reversed through the whole system of the universe.

It is, indeed, scarcely possible to imagine a more striking proof of the danger of extending, with too great certainty, a general law, than this instant conversion of attraction into repulsion without the addition of any new bodies, without any change in the nature of the bodies themselves, and a change of their circumstances so very slight, as to be absolutely indistinguishable, but for the opposite motions that result from it, with a change of their circumstances. After observing the gravity of bodies, at all heights of our atmosphere, and extending our survey through the wide spaces of our solar system, computing the tendency of the planets to the sun, and their disturbing forces, as they operate on each other, and finding the resulting motions exactly to correspond with those which we had predicted by theory; -in these circumstances, after an examination so extensive, if we had affirmed, as an universal law of matter, that, at all distances, bodies tend toward each other, we should have considered the wideness of the induction as justifying the affirmation; and yet, even in this case, we find, on the surface of our earth, in the mutual shocks of bodies, and in their very rest, sufficient evidence, that, in making the universal affirmation, we should have reasoned falsely. There is no theory, then, which, if applied to the explanation of new phenomena, is not, to a certain degree, conjectural; because it must proceed on the supposition, that what was true in certain circumstances, is true also in circumstances that have not been observed. It admits of certainty, only when it is applied to the very substances observed,—in the very circumstances observed, in which case, it may be strictly said to be nothing more than the application of a general term to the particulars, which we have before agreed to comprehend in it. Whatever is more than this is truly hypothetical,-the difference being, that we commonly give the name of hypothesis to cases, in which we suppose the intervention of some substance, of the existence of which, as present in the phenomenon, we have no direct proof, or of some additional quality of a substance before unobserved,—and the name of theory to cases, which do not suppose the existence of any substance, that is not actually observed, or of any quality that has not been actually observed, but merely the continuance, in certain new circumstances, of tendencies observed in other circumstances. Thus, if a planet were discovered revolving in the space which separates the orbits of any two planets at present known, were we to suppose of matter, in this new situation, that it would be subject to the same exact law of gravitation, to which the other planets were known to be subject, and to predict its place in the heavens, at any time, according to this law, we should be said to form a theory of its motions; as we should not take for granted any new quality of a substance, or the existence of any substance which was not evidently present, but only of tendencies observed before in other circumstances, analogous indeed, but not absolutely the same. We should be said to form an hypothesis on the subject, if, making the same prediction, as to its motions, and place in the heavens, at any given time, we were to ascribe the centripetal tendency, which confines it within its orbit, to the impulse of ether, or to any other mechanical cause. The terms, however, I must confess, though the distinction which I have now stated would be, in all cases, a very convenient one, are used very loosely, not in conversation merely, but in the writings of philosophers,-an hypothesis often meaning nothing more than a

theory, to which we have not given our assent, and a theory, an hypothesis which we have adopted, or still more, one which we have formed ourselves. A theory, then, even in that best sense, to which I wish it accurately confined, as often as it ventures a single hair-breadth beyond the line of former observation, may be wrong, as an hypothesis may be wrong. But, in a theory, in this sense of it, there are both less risk of error, and less extensive evil from error, than in an hypothesis. There is less risk of error, because we speak only of the properties of bodies, that must be allowed actually to exist; and the evil of error is, for the same reason, less extensive, since it must be confined to this single point; whereas, if we were to imagine falsely the presence of some third substance, our supposition might involve as many errors, as that substance has qualities; since we should be led to suppose, and expect, some or all of the other consequences, which usually attend it, when really present.

The practical conclusion to be drawn from all this very long discussion is, that we should use hypotheses to suggest and direct inquiry, not to terminate or supersede it; and that, in theorizing,-as the chance of error, in the application of a general law, diminishes, in proportion to the number of analogous cases, in which it is observed to hold,-we should not form any general proposition, till after as wide an induction, as it is possible for us to make; and, in the subsequent application of it to particulars, should never content ourselves, in any new circumstances, with the mere probability, however high, which this application of it affords, while it is possible for us to verify, or disprove it, by actual experiment.



For several Lectures, gentlemen, we have been employed in considering the objects that are to be had in view in Physical Inquiry in general, a clear conception of which seems to me as essential to the Philosophy of Mind as to the Philosophy of Matter. I should now proceed to apply these general remarks more particularly to our own science; but, before doing this, it may be of advantage to retrace slightly our steps in the progress already made.

All inquiry, with respect to the various substances in nature, we have seen, must regard them as they exist in space, or as they exist in time,—the inquiry, in the one case, being into their composition; the inquiry, in the other case, into the changes which they exhibit. The first of these views we found to be very simple, having for its object only the discovery of what is actually before us at the moment,-which, therefore, if we had been endowed with senses of greater delicacy and acuteness, we might have known without


any inquiry whatever. It is the investigation of the elements, or separate bodies, that exist together, in the substances which we considered, or rather that constitute the substances which we considered, by occupying the space which we assign to the one imaginary aggregate, and are regarded by us as one substance, not from any absolute unity which they have in nature, since the elementary atoms, however continuous or near, have an existence as truly separate and independent as if they had been created at the distance of worlds, but from a unity, that is relative only to our incapacity of distinguishing them as separate. It is to the imperfection of our senses, then, that this first division of Physical Inquiry owes its origin; and its most complete results could enable us to discover only what has been before our eyes from the moment of our birth.

The second division of inquiry,-that which relates to the successions of phenomena in time, we found, however, to have a different origin; since the utmost perfection of our mere senses could show us only what is, at the moment of perception, not what has been, nor what will be; and there is nothing in any qualities of bodies perceived by us, which, without experience, could enable us to predict the changes that are to occur in them. The foundation of all inquiry, with respect to phenomena as successive, we found to be that most important law, or original tendency, of our nature, in consequence of which we not merely perceive the changes exhibited to us at one particular moment, but from this perception, are led irresistibly to believe, that similar changes have constantly taken place in all similar circumstances, and will constantly take place, as often as the future circumstances shall be exactly similar to the present. We hence consider events, not as casually antecedent and consequent, but as invariably antecedent and consequent, or, in other words, as causes and effects; and we give the name of power to this permanent relation of the invariable antecedent to its invariable consequent. The powers of substances, then, concerning which so many vague, and confused, and mysterious notions prevail, are only another name for the substances themselves, in relation to other substances,-not any thing separate from them and intermediate,-as the form of a body, concerning which too, for many ages, notions as vague and mysterious prevailed, is not any thing different from the body, but is only the body itself, considered according to the relative position of its elements. Form is the relation of immediate proximity, which bodies bear to each other in space-power is the relation of immediate and uniform proximity, which events bear to each other in time; and the relation, far from being different, as is commonly supposed, when applied to matter and to spirit, is precisely the same in kind, whether the events, of which we think, be material or immaterial. It is of invariable antecedence that we speak alike in both cases, and of invariable antecedence only. When we say, that a magnet has the power of attracting iron, we mean only that a magnet cannot be brought near iron, without the instant motion of the iron towards it. When we say, in treating of mental influence, that man, in the ordinary circumstances of health, and when free from any foreign restraint, has the power of moving his hand, we mean only, that in these circumstances he cannot will to move his hand without its consequent motion. When we speak of the omnipotence of the Supreme of Beings, who is the fountain of all power, as he is the fountain of all existence, we mean only, that the universe arose at his command, as its instant consequence, and that whatever he wills to exist or perish, exists, or is no more.

This simple view of power, as the mere antecedent substance itself, in its relation to its immediate and invariable consequences, without the intervention of any mysterious tie,-since there surely can be nothing in nature, but all the substances which exist in nature,-it was necessary to illustrate at great length, in consequence of the very false notions that are generally, or, I may say, universally prevalent on the subject. This illustration, I am aware, must, to many of you, have appeared very tedious, and a sufficient exemplification of that license of exhausting occasionally your attention, and perhaps, too, your patience, of which I claimed the right of exercise, whenever it should appear to me necessary to make any important but abstract truth familiar to your mind. I shall not regret, however, any temporary feeling of weariness which I may have occasioned by dwelling on this great fundamental subject, if I have succeeded in making familiar to your minds the truths which I wished to impress on them, and have freed you from those false notions of occult and unintelligible agency in causes, as something different from the mere causes or antecedents themselves,-which appear to me to have retarded, in a very singular degree, the progress of philosophy,—not merely by habituating the mind to acquiesce in the use of language to which it truly affixes no meaning, though even this evil is one of very serious injury in its general effects, but by misdirecting its inquiries, and leading it from the simplicity of nature, in which every glance is truth, and every step is progress,-to bewilder itself with the verbal mysteries of the schools, where there is no refreshment of truth to the eye that is wearied with wandering only from shadow to shadow,-and where there is all the fatigue of continual progress, without the advance of a single step.

Even those philosophers, who have had the wisdom to perceive that man can never discover any thing in the phenomena of nature but a succession of events that follow each other in regular series, and who, accordingly, recommend the observation and arrangement of these regular antecedents and consequents as the only attainable objects of philosophy, yet found this very advice on the distinction of what they have termed efficient causes, as different from the physical causes, or simple antecedents, to which they advise us to devote our whole attention. There are certain secret causes, they say, continually operating in the production of every change which we observe, and causes which alone deserve the name of efficient; but they are at the same time careful to tell us, that although these causes are constantly operating before us, and are all which are truly acting before us, we must not hope that we shall ever be able to detect one of them; and indeed, the prohibition of every attempt to discover the efficient causes of phenomena,repeated in endless varieties of precept or reproof,-is the foundation of all their rules of philosophizing; as if the very information,-that what we are to consider exclusively in the phenomena of nature is far less important than what we are studiously to omit,-were not, of itself, more powerful in stimulating our curiosity to attempt the forbidden search, than any prohibition. could be in repressing it. "Felix qui potuit rerum cognoscere causas." This will for ever be the feeling of the inquirer while he thinks that there are any causes more than those which he has already investigated. Even Newton himself, that sagest of observers and reasoners, who could say, with the simplicity of pure philosophy, "Hypotheses non fingo," yet showed, as we have seen, by one of the most hypothetical of his Queries, that he was not exempt from the error which he wished to discourage-that inordinate

love of the unknown, which must always lead those who believe that there is something intermediate and undiscovered truly existing between events, to feel the anxious dissatisfaction of incomplete inquiry, in considering the mere antecedents and consequents which nature exhibits, and to turn, therefore, as if for comfort, to any third circumstance, which can be introduced, without obvious absurdity, as a sort of connecting link, between the pairs of events. To suppose that the mind should not have this disposition, would, indeed, be to suppose it void of that principle of curiosity, without which there can be no inquiry of any kind. He who could believe, that, between all the visible phenomena, there are certain invisible agencies continually operating, which have as real an existence as all that he perceives, and could yet content himself with numbering the visible phenomena, and giving them names, without any endeavour to discover the intervening powers, by which he is constantly surrounded, or at least to form some slight guess, as to that universal machinery, by which he conceived all the wonders of nature to be wrought, must be a being as different from the common intellectual beings of this earth, as the perfect sage of the Stoics from the frail creatures, of mingled vice and virtue, that live and err around us. That, in considering the phenomena of nature, we should confine our attention to the mere antecedents and consequents, which succeed each other in regular series, is unquestionably the soundest advice that can be given. But it is sound advice, for this reason more than any other, that the regular series is, in truth, all that constitutes the phenomena, and that to search for any thing more, is not to have an unattainable object in view, but to have no conceivable object whatever. Then only can the inquirer be expected to content himself with observing and classing the sequences, which nature presents to us spontaneously, or in obedience to our art, when he is convinced, that all the substances which exist in the universe-God and the things which he has created-are every thing which truly exists in the universe, to which nothing can be added which is not itself a new substance; that there can be nothing in the events of nature, therefore, but the antecedents and consequents which are present in them; and that these, accordingly, or nothing, are the very causes and effects which he is desirous of investigating.

After this examination of the notions connected with the uniform successions of events, our attention was next turned to the nature and origin of hypothetical inquiry, which we found reason to ascribe to the imperfection of our senses, that renders it impossible for us to know whether we have observed the whole train of sequences in any phenomenon, from our inability to distinguish the various elements that may be the subjects of minute changes unobserved.

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We are hence eager to supply, by a little guess-work of fancy, the parts unobserved, and suppose deficiencies in our observation where there may truly have been none; till at length, by this habitual process, every phenomenon becomes, to our imagination, the sign of something intermediate as its cause, the discovery of which is to be an explanation of the phenomenon. The mere succession of one event to another appears, to us, very difficult to be conceived, because it wants that intervening something which we have learned to consider as a cause; but there seems to be no longer any mystery, if we can only suppose something intervening between them, and can thus succeed in doubling the difficulty which we flatter ourselves with having removed; since, by the insertion of another link, we must now have two

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