According to this view of the subject, the confidence which we repose in Analogy rests ultimately on the evidence of experience; and hence an additional argument in favour of the former method of investigation when cautiously followed; as well as an additional proof of the imperceptible shades by which Experience and Analogy run into each other.

Nor is the utility of hypothetical theories confined to those cases in which they have been confirmed by subsequent researches: it may be equally great, where they have completely disappointed the expectations of their authors. Nothing, I think, can be juster than Hartley's remark, that " any hypothesis which possesses a sufficient degree of plausibility to account for a number of facts, helps us "to digest these facts in proper order, to bring new ones to fight, "and to make experimenta crucis for the sake of future inquirers."* Indeed, it has probably been in this way, that most discoveries have been made; for although a knowledge of facts must be prior to the formation of a legitimate theory; yet a hypothetical theory is generally the best guide to the knowledge of connected and of useful facts.

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The first conception of a hypothetical theory, it must always be remembered (if the theory possesses any plausibility whatever) presupposes a general acquaintance with the phenomena which it aims to account for; and it is by reasoning synthetically from the hypothesis and comparing the deductions with observation and experiment, that the cautious inquirer is gradually led, either to correct it in such a manner as to reconcile it with facts or finally to abandon it as an unfounded conjecture. Even in this latter case, an approach is made to the truth in the way of exclusion; while, at the same time, an accession is gained to that class of associated and kindred phenomena, which it is his object to trace to their parent stock.†

In thus apologizing for the use of hypotheses, I only repeat in a different form the precepts of Bacon and the comments of some of his most enlightened followers. "The prejudice against hypothe"ses which many people entertain, (says the late Dr. Gregory) is

* Observations on Man, Chap. i Prop v.

"Illud interim monemus; ut nemo animo concidat, aut quasi confundatur, si experi• menta, quibus incumbit, expectationi suae non respondean'. É enim quod succedit, magis complaceat, at quod non succedit, saepenumero non minus informat. Atque iud semper in animo tenendum, experimenta lucifera etiam adhuc magis, quam frue fera am bienda esse. Atque de literata experientia haec dicta sint; quae sagacitas potius est, et odoratio quaedam venatica, quam scientia." De Augm, Scient. Lib. v. Cap. ii.

[In the mean time I give this advice, that no one lose his courage, or be confounded. if the experiments on which he labours, do not succeed according to his expectation. When they do succeed, they are indeed more pleasing, but often when they do not, they are not therefore less instructive. And we should keep in mind, that experiments which throw some light are not less to be coveted than those which actually produce fruit This I speak of learned experiment, which is rather sagacity, and a scent as in the chase, than actual knowledge.].

"founded on the equivocal signification of a word. It is commonly "confounded with theory-but a hypothesis properly means the "supposition of a principle, of whose existence there is no proof "from experience, but which may be rendered more or less proba"ble by facts which are neither numerous enough, nor adequate to "infer its existence. When such hypotheses are proposed in the "modest and diffident manner that becomes mere suppositions or "conjectures, they are not only harmless, but even necessary for "establishing a just theory. They are the first rudiments or anti"cipations of Principles. Without these, there could not be use"ful observation, nor experiment, nor arrangement, because there "could be no motive or principle in the mind to form them. Hypo"theses then only become dangerous and censurable, when they "are imposed on us for just principles; because, in that case, they "put a stop to further inquiry, by leading the mind to acquiesce in

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99*

principles which may as probably be ill as well founded." Another eminent writer has apologized very ingeniously, and I think very philosophically, for the hypotheses and conjectures which are occasionally to be found in his own works. The author I mean is Dr. Stephen Hales, who, in the preface to the second volume of his Vegetable Statics, has expressed himself thus:

"In natural philosophy, we cannot depend on any mere specu"lations of the mind; we can only reason with any tolerable cer"tainty from proper data, such as arise from the united testimony of "many good and credible experiments.

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"Yet it seems not unreasonable, on the other hand, though not "far to indulge, to carry our reasonings a little farther than the plain evidence of experiments will warrant; for since at the utmost "boundaries of those things which we clearly know, a kind of "twilight is cast on the adjoining borders of Terra Incognita, it 66 seems reas asonable, in some degree, to indulge conjecture there; "otherwise we should make but very slow advances, either by "experiments or reasoning. For new experiments and discoveries "usually owe their first rise only to lucky guesses and probable "conjectures; and even disappointments in these conjectures often "lead to the thing sought for."

To these quotations I shall add two short extracts from Dr. Hooke (the contemporary or rather the predecessor of Newton,) whose acute and original remarks on this subject reflect the greater credit on his talents, that they were published at a period, when the learned body of which he was so illustrious an ornament, seem plainly to have been more disposed to follow the letter of some detached sentences, than to imbibe the general spirit of Bacon's logic.

"There may be use of method in the collecting of materials, "as well as in the employment of them; for there ought to be some end and aim, some predesigned module and theory; some "purpose in our experiments. And though this Society have

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*Lectures on the Duties and the Qualifications of a Physician.

"hitherto seemed to avoid and prohibit preconceived theories, and "deductions from particular and seemingly accidental experiments; "yet I humbly conceive, that such, if knowingly and judiciously "made, are matters of the greatest importance; as giving a charac"teristic of the aim, use, and signification thereof; and without "which, many, and possibly the most considerable particulars, are "passed over without regard and observation.*

Where the data on which our ratiocinations are founded are "uncertain and only conjectural, the conclusions or deductions "therefrom can at best be no other than probable, but still they "become more and more probable, as the consequences deduced "from them appear, upon examinations by trials and designed "observations, to be confirmed by fact or effect. So that the "effect is that which consummates the demonstration of the inven"tion; and the theory is only an assistant to direct such an inqui"sition, as may procure the demonstration of its existence or non"existence."

As an illustration of this last remark, Hooke mentions his anticipation of Jupiter's motion upon his axis, long before he was able, by means of a good telescope, to ascertain the fact. A much more remarkable instance, however, of his philosophical sagacity, occurs in his anticipation of that theory of the planetary motions, which, soon after, was to present itself, with increased and at length demonstrative evidence, to a still more inventive and powerful mind. This conjecture (which I shall state in his own words,) affords, of itself, a decisive reply to the undistinguishing censures which have so often been bestowed on the presumptuous vanity of attempting, by means of hypotheses, to penetrate into the secrets of nature.

"I will explain (says Hooke, in a communication to the Royal "Society in 1666) a system of the world very different from any "yet received. It is founded on the three following positions.

1. That all the heavenly bodies have not only a gravitation of "their parts to their own proper centre, but that they also mutually "attract each other within their spheres of action.

"2. That all bodies having a simple motion, will continue to "move in a straight line, unless continually deflected from it by "some extraneous force, causing them to describe a circle, an "ellipse, or some other curve.

"3. That this attraction is so much the greater as the bodies are CC nearer. As to the proportion in which those forces diminish by 66 an increase of distance, I own I have not discovered it, although “I have made some experiments to this purpose. I leave this to "others, who have time and knowledge sufficient for the task."

The argument in favour of Hypotheses might be pushed much farther, by considering the tentative or hypothetical steps by which,

Hooke's Posthumous Works, p. 280.

† Ibid. p. 537. For another extract from the same work, see note (S.)

the most cautious philosophers are often under the necessity of proceeding, in conducting inquiries strictly experimental. These cannot be better described than in the words of Boscovich, the slightest of whose logical hints are entitled to peculiar attention.— "In some instances, observations and experiments at once reveal to " us all that we wish to know. In other cases, we avail ourselves "of the aid of hypotheses ;-by which word, however, is to be under"stood, not fictions altoge her arbi'rary, bu supposi'i n‹ conformable "to experience or to analogy. By means of these, we are enabled "to supply the defects of our da a, and to conjecture or divine the "path to truth; always ready to abandon our hypothesis, when And indeed, "found to involve consequences inconsistent with fact. "in most cases, I conceive this to be the method best adapted to "physics; a science, in which the procedure of the inquirer may "be compared to that of a person attempting to decypher a letter "written in a secret character; and in which, legitimate theories "are generally the slow result of d sappointed essays, and of errors "which have led the way to their own detection."*

*De Solis ac Lunae Defectibus. the above passage, see note (T.)

Lond. 1760. pp. 211, 212. For the continuation of

Many remarks to the same purpose may be found in Bacon. The following happen at present to occur to my memory.

"Deo (formarum inditori et opifici) et fortasse angelis, competit, formas per affirmationem inmediate nosse, atque ab initio contemplationis. Sed certe supra hominem est ; cui tantum conceditur, procedere primo per negativas, et postremo loco desinere in Post rejectionem et exclusionem debitis affirmativas, post omnimodam exclusionem modis factam, secundo loco (tanquam in fundo) manebit (abeuntibus in fumum opinionibus volatilibus) forma affirmativa, solida, et vera, Atque hoc brevi dictu est, sed per mul tas ambages ad hoc pervenitur." (Nov. Org. Lib. II Aphor XV. XVI.)

..

Prudens interrogatio quasi dimidium scientiae. Idcirco quo amplior et certior fuerit anticipatio nostra ; eo magis directa et compendiosa erit investigatio." (De Aug. Scient. Lib. V. cap. 3.)

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Vaga experientia, et se tantum sequens, mera palpatio est, et homines potius stupe facit, quam informat." (Nov. Org. Dib. I. Aphor. C.)

The reader who wishes to prosecute farther this speculation concerning the use of hypotheses, may consult with advantage three short but interesting memoirs upon Method, by the late M. Le Sage of Geneva, which M. Prevost has annexed as a supplement to his Essais de Philosophie. That I may not be supposed, however, to acquiesce in all this author's views, I shall mention two strong objections o which some of them appear to me to be liable.

1. In treating of the method of Hypothesis, Le Sage uniformly contrasts it with that of Analogy, as if the two were radically distinct, and even opposite in their spirit; whereas it seems evident, that some perception of analogy must have given birth to every hypothesis which possesses a sufficient degree of plausibility to deserve farther examination.

It To God, the implanter and creator of forms, and perhaps to the angels, it is possible, to understand forms without any medium, by affirmation, and at the beginning of contemplation; but certainly this is beyond the faculties of man who is only permitted, first, to proceed by negatives, and then to conclude in affirmatives, after rejections of every sort.. After these rejections and exclusions have been properly effected, and false opinions have been evaporated into smoke, there will then remain as upon a sure foundation, a form affirmative, solid and true. Although this is stated easily and in a short space of time, yet we reach this point through many windings.

A well planned experiment is the half of knowledge The more complete and certain therefore has been our anticipation of truth, the more direct and compendious will be our investigation.

Vague experiment, without an object, is mere groping in the dark, and rather stupefies than instructs.]

Nor is it solely by the erroneous results of his own hypotheses, that the philosopher is assisted in the investigation of truth. Similar lights are often to be collected from the errors of his predecessors; and hence it is, that accurate histories of the different sciences may justly be ranked among the most effectual means of accelerating their future advancement. It was from a review of the endless and hopeless wanderings of preceding inquirers, that Bacon inferred the necessity of avoiding every beaten track; and it was this which encouraged him, with a confidence in his own powers amply justified by the event-to explore and to open a new path to the mysteries of nature: Inveniam viam, aut faciam. In this respect, the maturity of reason in the species is analogous to that in the individual; not the consequence of any sudden or accidental cause, but the fruit of reiterated disappointments correcting the mistakes of youth and inexperience. "There is no subject (says Fontenelle) "on which men ever come to form a reasonable opinion, till they "have once exhausted all the absurd views which it is possible to "take of it. What follies (he adds) should we not be repeating at "this day, if we had not been anticipated in so many of them by the "ancient philosophers !"-Those systems, therefore, which are false, are by no means to be regarded as altogether useless. That of Ptolemy (for example) as Bailly has well observed, is founded on a prejudice so natural and so unavoidable, that it may be considered as a necessary step in the progress of astronomical science; and if it had not been proposed in ancient times, it would infallibly have preceded, among the moderns, the system of Copernicus, and retarded the period of its discovery.

In what I have hitherto said in defence of the method of Hypothesis, I have confined myself entirely to its utility as an organ of investigation; taking all along for granted, that, till the principle assumed has been fairly inferred as a law of nature, from undoubted facts, none of the explanations which it affords are to be admitted as legitimate theories. Some of the advocates for this method have however gone much farther; asserting, that if a hypothesis be sufficient to account for all the phenomena in question, no other proof of its conformity to truth is necessary. Supposing (says "Dr. Hartley) the existence of the aether to be destitute of all direct "evidence, still, if it serves to explain and account for a great 66 variety of phenomena, it will, by this means, have an indirect "argument in its favour. Thus, we admit the key of a cypher to "be a true one, when it explains the cypher completely; and the "decypherer judges himself to approach to the true key, in pro66 portion as he advances in the explanation of the cypher; and this

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2. In applying the rules of Mathematical Method to Physics, he makes far too little allowance for the essential difference between the two sciences. This is more particularly remarkable in his observations on the aid to be derived, in investigating the laws of nature, from the method of Exclusions,-so happily employed by Frenicle de Bessy (a French mathematician of the 17th century) in the solution of some very difficult problems relating to numbers.-See note (U.)

VOL. II.

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