A dilemma may be raised of this kind. The universality of the law of causation—in other words, the uniform course of nature—is the fundamental principle on which all induction proceeds, the great premise on which all our science is founded. But if this law itself be the result only of experience, itself only a great instance of induction, so long as nature presents cases requiring investigation, where the causes are unknown to us, so long the law itself is imperfectly established. How, then, can this law be a guide and a premiss in the investigations of science, when those investigations are necessary to complete the proof of the law itself? How can this principle accompany and authorise every step we take in science, which itself needs confirmation so long as a process of induction remains to be performed? Or how can this law be established by a series of inductions, in making which it has been taken for granted?

Objections which wear the air of a quibble have often this advantage—they put our knowledge to the test. The obligation to find a complete answer clears up our own conceptions. The observations which Mr Mill makes on this point, we shall quote at length. They are taken from his chapter on the Evidence of the Law of Universal Causation; the views in which are as much distinguished for boldness as for precision.

After having said, that in all the several methods of induction the universality of the law of causation is assumed, he continues:—

"But is this assumption warranted? Doubtless (it may be said) most phenomena are connected as effects with some antecedent or cause—that is, are never produced unless some assignable fact has preceded them; but the very circumstance, that complicated processes of induction are sometimes necessary, shows that cases exist in which this regular order of succession is not apparent to our first and simplest apprehension. If, then, the processes which bring these cases within the same category with the rest, require that we should assume the universality of the very law which they do not at first sight appear to exemplify, is not this a real petitio principii? Can we prove a proposition by an argument which takes it for granted? And, if not so proved, on what evidence does it rest?

"For this difficulty, which I have purposely stated in the strongest terms it would admit of, the school of metaphysicians, who have long predominated in this country, find a ready salvo. They affirm that the universality of causation is a truth which we cannot help believing; that the belief in it is an instinct, one of the laws of our believing faculty. As the proof of this they say, and they have nothing else to say, that every body does believe it; and they number it among the propositions, rather numerous in their catalogue, which may be logically argued against, and perhaps cannot be logically proved, but which are of higher authority than logic, and which even he who denies in speculation, shows by his habitual practice that his arguments make no impression on himself.

"I have no intention of entering into the merits of this question, as a problem of transcendental metaphysics. But I must renew my protest against adducing, as evidence of the truth of a fact in external nature, any necessity which the human mind may be conceived to be under of believing it. It is the business of human intellect to adapt itself to the realities of things, and not to measure those realities by its own capacities of comprehension. The same quality which fits mankind for the offices and purposes of their own little life, the tendency of their belief to follow their experience, incapacitates them for judging of what lies beyond. Not only what man can know, but what he can conceive, depends upon what he has experienced. Whatever forms a part of all his experience, forms a part also of all his conceptions, and appears to him universal and necessary, though really, for aught he knows, having no existence beyond certain narrow limits. The habit, however, of philosophical analysis, of which it is the surest effect to enable the mind to command, instead of being commanded by, the laws of the merely passive part of its own nature, and which, by showing to us that things are not necessarily connected in fact because their ideas are connected in our minds, is able to loosen innumerable associations which reign despotically over the undisciplined mind; this habit is not without power even over those associations which the philosophical school, of which I have been speaking, regard as connate and instinctive. I am convinced that any one accustomed to abstraction and analysis, who will fairly exert his faculties for the purpose, will, when his imagination has once learned to entertain the notion, find no difficulty in conceiving that in some one, for instance, of the many firmaments into which sidereal astronomy now divides the universe, events may succeed one another at random, without any fixed law; nor can any thing in our experience, or in our mental nature, constitute a sufficient, or indeed any, reason for believing that this is nowhere the case. The grounds, therefore, which warrant us in rejecting such a supposition with respect to any of the phenomena of which we have experience, must be sought elsewhere than in any supposed necessity of our intellectual faculties.

"As was observed in a former place, the belief we entertain in the universality, throughout nature, of the law of cause and effect, is itself an instance of induction; and by no means one of the earliest which any of us, or which mankind in general, can have made. We arrive at this universal law by generalisation from many laws of inferior generality. The generalising propensity which, instinctive or not, is one of the most powerful principles of our nature, does not indeed wait for the period when such a generalisation becomes strictly legitimate. The mere unreasoning propensity to expect what has been often experienced, doubtless led men to believe that every thing had a cause, before they could have conclusive evidence of that truth. But even this cannot be supposed to have happened until many cases of causation, or, in other words, many partial uniformities of sequence, had become familiar. The more obvious of the particular uniformities suggest and prove the general uniformity; and that general uniformity, once established, enables us to prove the remainder of the particular uniformities of which it is made up. * * *

"With respect to the general law of causation, it does appear that there must have been a time when the universal prevalence of that law throughout nature could not have been affirmed in the same confident and unqualified manner as at present. There was a time when many of the phenomena of nature must have appeared altogether capricious and irregular, not governed by any laws, nor steadily consequent upon any causes. Such phenomena, indeed, were commonly, in that early stage of human knowledge, ascribed to the direct intervention of the will of some supernatural being, and therefore still to a cause. This shows the strong tendency of the human mind to ascribe every phenomenon to some cause or other; but it shows also that experience had not, at that time, pointed out any regular order in the occurrence of those particular phenomena, nor proved them to be, as we now know that they are, dependent upon prior phenomena as their proximate causes. There have been sects of philosophers who have admitted what they termed Chance as one of the agents in the order of nature by which certain classes of events were entirely regulated; which could only mean that those events did not occur in any fixed order, or depend upon uniform laws of causation. * * *

"The progress of experience, therefore, has dissipated the doubt which must have rested upon the universality of the law of causation, while there were phenomena which seemed to be sui generis; not subject to the same laws with any other class of phenomena, and not as yet ascertained to have peculiar laws of their own. This great generalisation, however, might reasonably have been, as it in fact was by all great thinkers, acted upon as a probability of the highest order, before there were sufficient grounds for receiving it as a certainty. For, whatever has been found true in innumerable instances, and never found to be false after due examination in any, we are safe in acting upon as universal provisionally, until an undoubted exception appears; provided the nature of the case be such that a real exception could scarcely have escaped our notice. When every phenomenon that we ever knew sufficiently well to be able to answer the question, had a cause on which it was invariably consequent, it was more rational to suppose that our inability to assign the causes of other phenomena arose from our ignorance, than that there were phenomena which were uncaused, and which happened accidentally to be exactly those which we had hitherto had no sufficient opportunity of studying."—Vol. II. p. 108.

Hypotheses.—Mr Mill's observations on the use of hypotheses in scientific investigation, except that they are characterized by his peculiar distinctness and accuracy of thought, do not differ from the views generally entertained by writers on the subject. We are induced to refer to the topic, to point out what seems to us a harsh measure dealt out to the undulatory theory of light—harsh when compared with the reception given to a theory of Laplace, having for its object to account for the origin of the planetary system.

We had occasion to quote a passage from Mr Mill, in which he remarks that the majority of scientific men seem not yet to have completely got over the difficulty of conceiving matter to act (contrary to the old maxim) where it is not; "for though," he says, "they have at last learned to conceive the sun attracting the earth without any intervening fluid, they cannot yet conceive the sun illuminating the earth without some such medium." But it is not only this difficulty (which doubtless, however, is felt) of conceiving the sun illuminating the earth without any medium by which to communicate its influence, which leads to the construction of the hypothesis, either of an undulating ether, or of emitted particles. The analogy of the other senses conducts us almost irresistibly to the imagination of some such medium. The nerves of sense are, apparently, in all cases that we can satisfactorily investigate, affected by contact, by impulse. The nerve of sight itself, we know, when touched or pressed upon, gives out the sensation of light. These reasons, in the first place, conduct us to the supposition of some medium, having immediate communication with the eye; which medium, though we are far from saying that its existence is established, is rendered probable by the explanation it affords of optical phenomena. At the same time it is evident that the hypothesis of an undulating ether, assumes a fluid or some medium, the existence of which cannot be directly ascertained. Thus stands the hypothesis of a luminiferous ether—in what must be allowed to be a very unsatisfactory condition. But a condition, we think, very superior to the astronomical speculation of Laplace, which Mr Mill, after scrutinizing the preceding hypothesis with the utmost strictness, is disposed to treat with singular indulgence.

"The speculation is," we may as well quote throughout Mr Mill's words, "that the atmosphere of the sun originally extended to the present limits of the solar system: from which, by the process of cooling, it has contracted to its present dimensions; and since, by the general principles of mechanics, the rotation of the sun and its accompanying atmosphere must increase as rapidly as its volume diminishes, the increased centrifugal force generated by the more rapid rotation, overbalancing the action of gravitation, would cause the sun to abandon successive rings of vaporous matter, which are supposed to have condensed by cooling, and to have become our planets.

"There is in this theory," Mr Mill proceeds, "no unknown substance introduced upon supposition, nor any unknown property or law ascribed to a known substance. The known laws of matter authorize us to suppose, that a body which is constantly giving out so large an amount of heat as the sun is, must be progressively cooling, and that by the process of cooling it must contract; if, therefore, we endeavour, from the present state of that luminary, to infer its state in a time long past, we must necessarily suppose that its atmosphere extended much further than at present, and we are entitled to suppose that it extended as far as we can trace those effects which it would naturally leave behind it on retiring; and such the planets are. These suppositions being made, it follows from known laws that successive zones of the solar atmosphere would be abandoned; that these would continue to revolve round the sun with the same velocity as when they formed part of his substance, and that they would cool down, long before the sun himself, to any given temperature, and consequently to that at which the greater part of the vaporous matter of which they consisted would become liquid or solid. The known law of gravitation would then cause them to agglomerate in masses, which would assume the shape our planets actually exhibit; would acquire, each round its own axis, a rotatory movement; and would in that state revolve, as the planets actually do, about the sun, in the same direction with the sun's rotation, but with less velocity, and each of them in the same periodic time which the sun's rotation occupied when his atmosphere extended to that point; and this also M. Comte has, by the necessary calculations, ascertained to be true, within certain small limits of error. There is thus in Laplace's theory nothing hypothetical; it is an example of legitimate reasoning from a present effect to its past cause, according to the known laws of that case; it assumes nothing more than that objects which really exist, obey the laws which are known to be obeyed by all terrestrial objects resembling them."—Vol. II. p. 27.

Now, it seems to us that there is quite as much of hypothesis in this speculation of Laplace as in the undulatory theory of light. This atmosphere of the sun extending to the utmost limits of our planetary system! What proof have we that it ever existed? what possible grounds have we for believing, what motive even for imagining such a thing, but the very same description of proof given and rejected for the existence of a luminiferous ether—namely, that it enables us to explain certain events supposed to result from it? Nor is the thing here imagined any the less a novelty, because it bears the old name of an atmosphere. An atmosphere containing in itself all the various materials which compose our earth, and whatever else may enter into the composition of the other planets, is as violent a supposition as an ether, not perceptible to the senses except by its influence on the nerves of sight. And this cooling down of the sun! What fact in our experience enables us to advance such a supposition? We might as well say that the sun was getting hotter every year, or harder or softer, or larger or smaller. Surely Mr Mill could not have been serious when he says, that "the known laws of matter authorize us to suppose, that a body which is constantly giving out so large an amount of heat as the sun is, must be progressively cooling"—knowing, as we do, as little how the sun occasions heat as how it produces light. Neither can it be contended that because no absolutely new substance, or new property of matter, is introduced, but a fantastic conception is framed out of known substances and known properties, that therefore there is less of rash conjecture in the supposition. In fine, it must be felt by every one who reads the account of this speculation of Laplace, that the only evidence which produces the least effect upon his mind, is the corroboration which it receives from the calculations of the mathematician—a species of proof which Mr Mill himself would not estimate very highly.

Many are the topics which are made to reflect a new light as Mr Mill passes along his lengthened course; we might quote as instances, his chapters on Analogy and the Calculation of Chances: and many are the grave and severe discussions that would await us were we to proceed to the close of his volumes, especially to that portion of his work where he applies the canons of science to investigations which relate to human nature and the characters of men. But enough for the present. We repeat, in concluding, the same sentiment that we expressed at the commencement, that such a work as this goes far to redeem the literature of our age from the charge of frivolity and superficiality. Those who have been trained in a different school of thinking, those who have adopted the metaphysics of the transcendental philosophy, will find much in these volumes to dissent from; but no man, be his pretensions or his tenets what they may, who has been accustomed to the study of philosophy, can fail to recognize and admire in this author that acute, patient, enlarged, and persevering thought, which gives to him who possesses it the claim and right to the title of philosopher. There are few men who—applying it to his own species of excellence—might more safely repeat the Io sono anche! of the celebrated Florentine.