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chanics or to physics, the abstractions which are necessary in the theory, must always leave out circumstances which are essentially connected with the effect. In demonstrating, for example, the property of the lever, we abstract entirely from its own weight, and consider it as an inflexible mathematical line;-suppositions with which the fact cannot possibly correspond; and for which, of course, allowances (which nothing but physical experience can enable us to judge of,) must be made in practice.*

Next to practical geometry, properly so called, one of the easiest applications of mathematical theory occurs in those branches of (atoptours optics which are distinguished by the name of catoptrics and diop- optries. trics. In these, the physical principles from which we reason are few and precisely definite, and the rest of the process is as purely geometrical as the Elements of Euclid.

In that part of astronomy, too, which relates solely to the pheno- Astronom mena, without any consideration of physical causes, our reasonings ical phe are purely geometrical. The data indeed on which we proceed nomena. must have been previously ascertained by observation; but the inferences we draw from these are connected with them by mathematical demonstration, and are accessible to all who are acquainted with the theory of spherics.

In physical astronomy, the law of gravitation becomes also a prin- Thysel ciple or datum in our reasonings; but as in the celestial phenomena, astronomy

it is disengaged from the effects of the various other causes which are combined with it near the surface of our planet, this branch of .physics, as it is of all the most sublime and comprehensive in its objects, so it seems, in a greater degree than any other, to open a fair and advantageous field for mathematical ingenuity.

In the instances which have been last mentioned, the evidence of our conclusions resolves ultimately not only into that of sense, but into another law of belief formerly mentioned; that which leads us

will happen

to expect the continuance, in future, of the established order of phy. belief thing sical phenomena. A very striking illustration of this presents itself in the computations of the astronomer; on the faith of which he predicts, with the most perfect assurance, many centuries before they happen, the appearances which the heavenly bodies are to exhibit. The same fact is assumed in all our conclusions in natural


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future, a They have in

philosophy; and something extremely analogous to it in all our con-time past. clusions concerning human affairs. They relate, in both cases, not to necessary connexions, but to probable or contingent events; of which (how confidently soever we may expect them to take place,) the failure is by no means perceived to be impossible. Such conclusions, therefore, differ essentially from those to which we are led by the demonstrations of pure mathematics, which not only command our assent to the theorems they establish, but satisfy us that the contrary suppositions are absurd.

* See note (H.)

These examples may suffice to convey a general idea of the distinction between demonstrative and probable evidence; and I purposely borrow them from sciences where the two are brought into mmediate contrast with each other, and where the authority of both has hitherto been equally undisputed.

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Before prosecuting any farther the subject of probable evidence, some attention seems to be due, in the first place, to the grounds of that fundamental supposition on which it proceeds, the stability of the order of nature. Of this important subject, accordingly, I propose to treat at some length.

I HAVE already taken notice of a remarkable principle of the mind, (whether coeval with the first exercise of its powers, or the gradual result of habit, it is not at present material to inquire,) in consequence of which, we are irresistibly led to apply to future events the results of our past experience. In again resuming the subject, I do not mean to add any thing to what was then stated concerning the origin or the nature of this principle; but shall confine myself to a few reflections on that established order in the suc cession of events, which it unconsciously assumes as a fact; and which, if it were not real, would render human life a continued series of errours and disappointments. In any incidental remarks that may occur on the principle itself, I shall consider its existence as a thing universally acknowledged, and shall direct my attention • poop, chiefly to its practical effects;-effects which will be found to ex

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tend equally to the theories of the learned, and to the prejudices of the vulgar. The question with regard to its origin is, in truth, a problem of mere curiosity; for of its actual influence on our belief, and on our conduct, no doubts have been suggested by the most sceptical writers.


Continuation of the Subject. Of that Permanence or Stability in the Order of Nature, which is presupposed in our Reasonings concerning Contingent Truths.

Before entering, however, upon the following argument, it may experta-not be superfluous to observe, with respect to this expectation, that, in whatever manner it at first arises, it cannot fail to be mightily conon strength-firmed and strengthened by habits of scientific research; the tenby sci-dency of which is to familiarize us more and more with the simplitific-city and the uniformity of physical laws, by gradually reconciling


extends, those

with them, as our which we had previously been disposed to consider in the light of exceptions. It is thus that, when due allowances are made for the different circumstances of the two events, the ascent of smoke appears to be no less a proof of the law of gravitation than the fall of a stone. This simplification and generalization of the laws of nature is one of the greatest pleasures which philosophy yields; and the growing confi

dence with which it is anticipated, forms one of the chief incentives to philosophical pursuits. Few experiments perhaps, in physics, afford more exquisite delight to the novice, or throw a stronger light on the nature and object of that science, than when he sees, for the first time, the guinea and the feather drop together in the exhausted receiver.

In the language of modern science, the established order in the succession of physical events is commonly referred (by a sort of figure or the

* So likewise Halley, in his Latin verses prefixed to Newton's Principia :

"En tibi norma poli, et divae libramina molis,

Computus en Jovis; et quas, dum primordia rerum,
Pangeret, omniparens leges violare Creator





speaking extremely convenient from its conciseness, but is apt to suggest to the fancy a groundless, and, indeed, absurd analogy between the material and the moral worlds. As the order of society results from the laws prescribed by the legislator, so the order of the universe is conceived to result from certain laws established by the Deity. Thus, it is customary to say, that the fall of heavy bodies towards the earth's surface, the ebbing and flowing of the sea, and the motions of the planets in their orbits, are consequences of the law of gravitation. But although, in one sense, this may be abundantly accurate, it ought always to be kept in view, that it is not a literal but a metaphorical statement of the truth; a statement somewhat analogous to that poetical expression in the sacred writings, in which God is said to have given his decree to the seas, "that they should not pass his commandment." In those political associations from which the metaphor is borrowed, the laws are ad-s -Laws appli dressed to rational and voluntary agents, who are able to comprehend their meaning, and to regulate their conduct accordingly; cable only to whereas, in the material universe, the of our observation are understood by all men to be unconscious and passive, (that is, pupient are understood to be unchangeable in their state, without the influ- beings ence of some foreign and external force) and, consequently, the order so admirably maintained, amidst all the various changes which they actually undergo, not only implies intelligence in its first conception, but implies, in its continued existence, the incessant agency of power, executing the purposes of wise design. If the word law, therefore, be, in such instances, literally interpreted, it must mean a uniform mode of operation, prescribed by the Deity to himself; and it has accordingly been explained in this sense by some of our best philosophical writers, particularly by Dr. Clarke.* In employing, however, the word with an exclusive reference to experimental philosophy, it is more correctly logical to consider it as merely a statement of some general fact with respect to the order of nature ;— a fact which has been found to hold uniformly in our past experience, and on the continuance of which, in future, the constitution of our mind determines us confidently to rely.



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After what has been already said, it is hardly necessary to take notice of the absurdity of that opinion, or rather of that mode of General law, speaking, which seems to refer the order of the universe to general laws operating as efficient causes. Absurd, however, as it is, there operating is reason to suspect, that it has, with many, had the effect of keeping efficient the Deity out of view, while they were studying his works. To an incautious use of the same very equivocal phrase, may be traced the bewildering obscurity in the speculations of some eminent French writers, concerning its metaphysical import. Even the great Montesquieu, in the very first chapter of his principal work, has lost himself in a fruitless attempt to explain its meaning, when, by a simple statement of the essential distinction between its literal and its metaphorical acceptations, he might have at once cleared up the mystery. After telling us that "laws, in their most extensive sig"nification, are the necessary relations (les rapports nécessaires) which arise from the nature of things, and that, in this sense, all "beings have their laws;-that the Deity has his laws; the mate"rial world its laws; intelligences superiour to man their laws; the "brutes their laws; man his laws;"-he proceeds to remark, "That "the moral world is far from being so well governed, as the mate"rial; for the former, although it has its laws, which are invaria"ble, does not observe these laws so constantly as the latter." It is evident that this remark derives whatever plausibility it pos sesses from a play upon words; from confounding moral laws with physical; or, in plainer terms, from confounding laws which are addressed by a legislator to intelligent beings, with those general conclusions concerning the established order of the universe, to which, when legitimately inferred from an induction sufficiently extensive, philosophers have metaphorically applied the title of Laws of NaIn the one case, the conformity of the law with the nature of things does not at all depend on its being observed or not, but on the reasonableness and moral obligation of the law. In the other case, the very definition of the word law supposes that it applies universally; in so much that, if it failed in one single instance, it would cease to be a law. It is, therefore, a mere quibble to say, that the laws of the material world are better observed than those of the moral; the meaning of the word law, in the two cases to which it is here applied, being so totally different, as to render the comparison or contrast, in the statement of which it is involved, altogether illusory and sophistical. Indeed, nothing more is necessary to strip the proposition of every semblance of plausibility, but an attention to this verbal ambiguity.*


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this classical uthom.

This metaphorical employment of the word law, to express a general fact, although it does not appear to have been adopted in the technical phraseology of ancient philosophy, is not unusual among

I do not recollect any instance in the writings of Montesquieu, where he has reasoned more vaguely than in this chapter; and yet I am inclined to believe, that few chapters in the Spirit of Laws have been more admired. "Montesquieu (says a French writer) paroissoit à Thomas le premier des écrivains, pour la force et l'éntendue des idées, pour

the classical writers, when speaking of those physical arrangements, whether on the earth or in the heavens, which continue to exhibit the same appearance from age to age.

"Hic segetes, illic veniunt felicius uvae:
Arborei foetus alibi, atque injussa virescunt
Gramina. Nonne vides, croceos ut Tmolus odores,
India mittit ebur, molles sua thura Sabaei?
At Chalybes nudi ferrum, virosaque Pontus
Castorea, Eliadum palmas Epiros equarum ?
Continuo has leges, aeternaque foedera certis
Imposuit natura locis."*

The same metaphor occurs in another passage of the Georgics where the poet describes the regularity which is exhibited in the economy of the bees:

"Solae communes natos, consortia tecta

Urbis habent, magnisque agitant sub legibus aevum."t

The following lines from Ovid's account of the Pythagorean philosophy, are still more in point:

• Virg. I. Georg. 60.

la multitude, la profondeur, la nouveauté des rapports. Il est incroyable (disoit-il) tout ce que Montesquieu a fait appercevoir dans ce mot si court, le mot Loi." (Nouveau

Diction. Historique, Art. Thomas. Lyon, 1804.)

For some important remarks on the distinction between moral and physical laws, see Dr. Ferguson's Institutes of Moral Philosophy, last edit.

Ere virgin earth first feel the invading share
The genius of the place demands thy care:

The culture, clime, the winds, and changeful skies,
And what each region bears and what denies.
Here golden harvests wave, there vineyards glow,
Fruit bends the bough, or herbs unbidden grow..
Her Saffron Tmolus, Ind her Ivory boasts,
Spice wings the gale round Saba's balmy coasts;
The naked Chalybes their iron yield,

The powerful Castor scents the Pontic field.
While fam'd Epirus rears the equestrian breed,
Born for the palm that crowns the Olympic steed,
In stated regions, from the Eternal Cause,
Such nature's compact and unbroken laws.

+ Georg. IV. 153.



They, they alone a common race supply,
And dwell in towns beneath the public eye,
Love their own household, aid their country's cause,
Securely live beneath established laws.


[Montesquieu appeared to Thomas the first of writers, in the force and extent of his ideas; in the multitude, the depth and the novelty of his statements. It is incredible, said he, how much Montesquieu has enabled us to understand in the simple word, law.]

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