Three years have now elapsed since I finished the treatise containin all these matters; and I was beginning to revise it, with the view t put it into the hands of a printer, when I learned that persons to who I greatly defer, and whose authority over my actions is hardly les influential than is my own reason over my thoughts, had condemned certain doctrine in physics, published a short time previously b another individual to which I will not say that I adhered, but onl that, previously to their censure I had observed in it nothing which could imagine to be prejudicial either to religion or to the state, an nothing therefore which would have prevented me from giving expressio to it in writing, if reason had persuaded me of its truth; and this le me to fear lest among my own doctrines likewise some one might be foun in which I had departed from the truth, notwithstanding the great car I have always taken not to accord belief to new opinions of which I ha not the most certain demonstrations, and not to give expression t aught that might tend to the hurt of any one. This has been sufficien to make me alter my purpose of publishing them; for although th reasons by which I had been induced to take this resolution were ver strong, yet my inclination, which has always been hostile to writin books, enabled me immediately to discover other consideration sufficient to excuse me for not undertaking the task. And thes reasons, on one side and the other, are such, that not only is it i some measure my interest here to state them, but that of the public perhaps, to know them

I have never made much account of what has proceeded from my own mind and so long as I gathered no other advantage from the method I emplo beyond satisfying myself on some difficulties belonging to th speculative sciences, or endeavoring to regulate my actions accordin to the principles it taught me, I never thought myself bound to publis anything respecting it. For in what regards manners, every one is s full of his own wisdom, that there might be found as many reformers a heads, if any were allowed to take upon themselves the task of mendin them, except those whom God has constituted the supreme rulers of hi people or to whom he has given sufficient grace and zeal to b prophets; and although my speculations greatly pleased myself, believed that others had theirs, which perhaps pleased them still more But as soon as I had acquired some general notions respecting physics and beginning to make trial of them in various particular difficulties had observed how far they can carry us, and how much they differ fro the principles that have been employed up to the present time, believed that I could not keep them concealed without sinnin grievously against the law by which we are bound to promote, as far a in us lies, the general good of mankind. For by them I perceived it t be possible to arrive at knowledge highly useful in life; and in roo of the speculative philosophy usually taught in the schools, t discover a practical, by means of which, knowing the force and actio of fire, water, air the stars, the heavens, and all the other bodie that surround us, as distinctly as we know the various crafts of ou artisans, we might also apply them in the same way to all the uses t which they are adapted, and thus render ourselves the lords an possessors of nature. And this is a result to be desired, not only i order to the invention of an infinity of arts, by which we might b enabled to enjoy without any trouble the fruits of the earth, and al its comforts, but also and especially for the preservation of health which is without doubt, of all the blessings of this life, the firs and fundamental one; for the mind is so intimately dependent upon th condition and relation of the organs of the body, that if any means ca ever be found to render men wiser and more ingenious than hitherto, believe that it is in medicine they must be sought for. It is tru that the science of medicine, as it now exists, contains few thing whose utility is very remarkable: but without any wish to depreciat it, I am confident that there is no one, even among those whos profession it is, who does not admit that all at present known in it i almost nothing in comparison of what remains to be discovered; and tha we could free ourselves from an infinity of maladies of body as well a of mind, and perhaps also even from the debility of age, if we ha sufficiently ample knowledge of their causes, and of all the remedie provided for us by nature. But since I designed to employ my whol life in the search after so necessary a science, and since I had falle in with a path which seems to me such, that if any one follow it h must inevitably reach the end desired, unless he be hindered either b the shortness of life or the want of experiments, I judged that ther could be no more effectual provision against these two impediments tha if I were faithfully to communicate to the public all the little might myself have found, and incite men of superior genius to strive t proceed farther, by contributing, each according to his inclination an ability, to the experiments which it would be necessary to make, an also by informing the public of all they might discover, so that, b the last beginning where those before them had left off, and thu connecting the lives and labours of many, we might collectively procee much farther than each by himself could do

I remarked, moreover, with respect to experiments, that they becom always more necessary the more one is advanced in knowledge; for, a the commencement, it is better to make use only of what i spontaneously presented to our senses, and of which we cannot remai ignorant, provided we bestow on it any reflection, however slight, tha to concern ourselves about more uncommon and recondite phenomena: th reason of which is, that the more uncommon often only mislead us s long as the causes of the more ordinary are still unknown; and th circumstances upon which they depend are almost always so special an minute as to be highly difficult to detect. But in this I have adopte the following order: first, I have essayed to find in general th principles, or first causes of all that is or can be in the world without taking into consideration for this end anything but God himsel who has created it, and without educing them from any other source tha from certain germs of truths naturally existing in our minds In th second place, I examined what were the first and most ordinary effect that could be deduced from these causes; and it appears to me that, i this way, I have found heavens, stars, an earth, and even on the eart water, air, fire, minerals, and some other things of this kind, whic of all others are the most common and simple, and hence the easiest t know. Afterwards when I wished to descend to the more particular, s many diverse objects presented themselves to me, that I believed it t be impossible for the human mind to distinguish the forms or species o bodies that are upon the earth, from an infinity of others which migh have been, if it had pleased God to place them there, or consequentl to apply them to our use, unless we rise to causes through thei effects, and avail ourselves of many particular experiments Thereupon, turning over in my mind I the objects that had ever bee presented to my senses I freely venture to state that I have neve observed any which I could not satisfactorily explain by the principle had discovered. But it is necessary also to confess that the power o nature is so ample and vast, and these principles so simple an general, that I have hardly observed a single particular effect which cannot at once recognize as capable of being deduced in man differen modes from the principles, and that my greatest difficulty usually i to discover in which of these modes the effect is dependent upon them for out of this difficulty cannot otherwise extricate myself than b again seeking certain experiments, which may be such that their resul is not the same, if it is in the one of these modes at we must explai it, as it would be if it were to be explained in the other. As to wha remains, I am now in a position to discern, as I think, with sufficien clearness what course must be taken to make the majority thos experiments which may conduce to this end: but I perceive likewis that they are such and so numerous, that neither my hands nor m income, though it were a thousand times larger than it is, would b sufficient for them all; so that according as henceforward I shall hav the means of making more or fewer experiments, I shall in the sam proportion make greater or less progress in the knowledge of nature This was what I had hoped to make known by the treatise I had written and so clearly to exhibit the advantage that would thence accrue to th public, as to induce all who have the common good of man at heart, tha is, all who are virtuous in truth, and not merely in appearance, o according to opinion, as well to communicate to me the experiments the had already made, as to assist me in those that remain to be made

But since that time other reasons have occurred to me, by which I hav been led to change my opinion, and to think that I ought indeed to g on committing to writing all the results which I deemed of any moment as soon as I should have tested their truth, and to bestow the sam care upon them as I would have done had it been my design to publis them. This course commended itself to me, as well because I thu afforded myself more ample inducement to examine them thoroughly, fo doubtless that is always more narrowly scrutinized which we believ will be read by many, than that which is written merely for our privat use (and frequently what has seemed to me true when I first conceive it, has appeared false when I have set about committing it to writing) as because I thus lost no opportunity of advancing the interests of th public, as far as in me lay, and since thus likewise, if my writing possess any value, those into whose hands they may fall after my deat may be able to put them to what use they deem proper. But I resolve by no means to consent to their publication during my lifetime, les either the oppositions or the controversies to which they might giv rise, or even the reputation, such as it might be, which they woul acquire for me, should be any occasion of my losing the time that I ha set apart for my own improvement. For though it be true that every on is bound to promote to the extent of his ability the good of others and that to be useful to no one is really to be worthless, yet it i likewise true that our cares ought to extend beyond the present, and i is good to omit doing what might perhaps bring some profit to th living, when we have in view the accomplishment of other ends that wil be of much greater advantage to posterity. And in truth, I am quit willing it should be known that the little I have hitherto learned i almost nothing in comparison with that of which I am ignorant, and t the knowledge of which I do not despair of being able to attain; for i is much the same with those who gradually discover truth in th sciences, as with those who when growing rich find less difficulty i making great acquisitions, than they formerly experienced when poor i making acquisitions of much smaller amount. Or they may be compared t the commanders of armies, whose forces usually increase in proportio to their victories, and who need greater prudence to keep together th residue of their troops after a defeat than after a victory to tak towns and provinces. For he truly engages in battle who endeavors t surmount all the difficulties and errors which prevent him fro reaching the knowledge of truth, and he is overcome in fight who admit a false opinion touching a matter of any generality and importance, an he requires thereafter much more skill to recover his former positio than to make great advances when once in possession of thoroughl ascertained principles. As for myself, if I have succeeded i discovering any truths in the sciences (and I trust that what i contained in this volume I will show that I have found some), I ca declare that they are but the consequences and results of five or si principal difficulties which I have surmounted, and my encounters wit which I reckoned as battles in which victory declared for me. I wil not hesitate even to avow my belief that nothing further is wanting t enable me fully to realize my designs than to gain two or three simila victories; and that I am not so far advanced in years but that according to the ordinary course of nature, I may still have sufficien leisure for this end. But I conceive myself the more bound to husban the time that remains the greater my expectation of being able t employ it aright, and I should doubtless have much to rob me of it were I to publish the principles of my physics: for although they ar almost all so evident that to assent to them no more is needed tha simply to understand them, and although there is not one of them o which I do not expect to be able to give demonstration, yet, as it i impossible that they can be in accordance with all the diverse opinion of others, I foresee that I should frequently be turned aside from m grand design, on occasion of the opposition which they would be sure t awaken

It may be said, that these oppositions would be useful both in makin me aware of my errors, and, if my speculations contain anything o value, in bringing others to a fuller understanding of it; and stil farther, as many can see better than one, in leading others who are no beginning to avail themselves of my principles, to assist me in tur with their discoveries. But though I recognize my extreme liability t error, and scarce ever trust to the first thoughts which occur to me yet-the experience I have had of possible objections to my view prevents me from anticipating any profit from them. For I have alread had frequent proof of the judgments, as well of those I esteeme friends, as of some others to whom I thought I was an object o indifference, and even of some whose malignancy and envy would, I knew determine them to endeavor to discover what partiality concealed fro the eyes of my friends. But it has rarely happened that anything ha been objected to me which I had myself altogether overlooked, unless i were something far removed from the subject: so that I have never me with a single critic of my opinions who did not appear to me eithe less rigorous or less equitable than myself. And further, I have neve observed that any truth before unknown has been brought to light by th disputations that are practised in the schools; for while each strive for the victory, each is much more occupied in making the best of mer verisimilitude, than in weighing the reasons on both sides of th question; and those who have been long good advocates are no afterwards on that account the better judges

As for the advantage that others would derive from the communication o my thoughts, it could not be very great; because I have not yet so fa prosecuted them as that much does not remain to be added before the can be applied to practice. And I think I may say without vanity, tha if there is any one who can carry them out that length, it must b myself rather than another: not that there may not be in the worl many minds incomparably superior to mine, but because one cannot s well seize a thing and make it one's own, when it has been learned fro another, as when one has himself discovered it. And so true is this o the present subject that, though I have often explained some of m opinions to persons of much acuteness, who, whilst I was speaking appeared to understand them very distinctly, yet, when they repeate them, I have observed that they almost always changed them to such a extent that I could no longer acknowledge them as mine. I am glad, b the way, to take this opportunity of requesting posterity never t believe on hearsay that anything has proceeded from me which has no been published by myself; and I am not at all astonished at th extravagances attributed to those ancient philosophers whose ow writings we do not possess; whose thoughts, however, I do not on tha account suppose to have been really absurd, seeing they were among th ablest men of their times, but only that these have been falsel represented to us. It is observable, accordingly, that scarcely in single instance has any one of their disciples surpassed them; and I a quite sure that the most devoted of the present followers of Aristotl would think themselves happy if they had as much knowledge of nature a he possessed, were it even under the condition that they should neve afterwards attain to higher. In this respect they are like the iv which never strives to rise above the tree that sustains it, and whic frequently even returns downwards when it has reached the top; for i seems to me that they also sink, in other words, render themselves les wise than they would be if they gave up study, who, not contented wit knowing all that is intelligibly explained in their author, desire i addition to find in him the solution of many difficulties of which h says not a word, and never perhaps so much as thought. Their fashio of philosophizing, however, is well suited to persons whose abilitie fall below mediocrity; for the obscurity of the distinctions an principles of which they make use enables them to speak of all thing with as much confidence as if they really knew them, and to defend al that they say on any subject against the most subtle and skillful without its being possible for any one to convict them of error. I this they seem to me to be like a blind man, who, in order to fight o equal terms with a person that sees, should have made him descend t the bottom of an intensely dark cave: and I may say that such person have an interest in my refraining from publishing the principles of th philosophy of which I make use; for, since these are of a kind th simplest and most evident, I should, by publishing them, do much th same as if I were to throw open the windows, and allow the light of da to enter the cave into which the combatants had descended. But eve superior men have no reason for any great anxiety to know thes principles, for if what they desire is to be able to speak of al things, and to acquire a reputation for learning, they will gain thei end more easily by remaining satisfied with the appearance of truth which can be found without much difficulty in all sorts of matters than by seeking the truth itself which unfolds itself but slowly an that only in some departments, while it obliges us, when we have t speak of others, freely to confess our ignorance. If, however, the prefer the knowledge of some few truths to the vanity of appearin ignorant of none, as such knowledge is undoubtedly much to b preferred, and, if they choose to follow a course similar to mine, the do not require for this that I should say anything more than I hav already said in this discourse. For if they are capable of makin greater advancement than I have made, they will much more be able o themselves to discover all that I believe myself to have found; sinc as I have never examined aught except in order, it is certain that wha yet remains to be discovered is in itself more difficult and recondite than that which I have already been enabled to find, and th gratification would be much less in learning it from me than i discovering it for themselves. Besides this, the habit which they wil acquire, by seeking first what is easy, and then passing onward slowl and step by step to the more difficult, will benefit them more than al my instructions. Thus, in my own case, I am persuaded that if I ha been taught from my youth all the truths of which I have since sough out demonstrations, and had thus learned them without labour, I shoul never, perhaps, have known any beyond these; at least, I should neve have acquired the habit and the facility which I think I possess i always discovering new truths in proportion as I give myself to th search. And, in a single word, if there is any work in the world whic cannot be so well finished by another as by him who has commenced it it is that at which I labour

It is true, indeed, as regards the experiments which may conduce t this end, that one man is not equal to the task of making them all; bu yet he can advantageously avail himself, in this work, of no hand besides his own, unless those of artisans, or parties of the same kind whom he could pay, and whom the hope of gain (a means of grea efficacy) might stimulate to accuracy in the performance of what wa prescribed to them. For as to those who, through curiosity or a desir of learning, of their own accord, perhaps, offer him their services besides that in general their promises exceed their performance, an that they sketch out fine designs of which not one is ever realized they will, without doubt, expect to be compensated for their trouble b the explication of some difficulties, or, at least, by compliments an useless speeches, in which he cannot spend any portion of his tim without loss to himself. And as for the experiments that others hav already made, even although these parties should be willing o themselves to communicate them to him (which is what those who estee them secrets will never do), the experiments are, for the most part accompanied with so many circumstances and superfluous elements, as t make it exceedingly difficult to disentangle the truth from it adjuncts--besides, he will find almost all of them so ill described, o even so false (because those who made them have wished to see in the only such facts as they deemed conformable to their principles), that if in the entire number there should be some of a nature suited to hi purpose, still their value could not compensate for the time what woul be necessary to make the selection. So that if there existed any on whom we assuredly knew to be capable of making discoveries of th highest kind, and of the greatest possible utility to the public; an if all other men were therefore eager by all means to assist him i successfully prosecuting his designs, I do not see that they could d aught else for him beyond contributing to defray the expenses of th experiments that might be necessary; and for the rest, prevent hi being deprived of his leisure by the unseasonable interruptions of an one. But besides that I neither have so high an opinion of myself a to be willing to make promise of anything extraordinary, nor feed o imaginations so vain as to fancy that the public must be muc interested in my designs; I do not, on the other hand, own a soul s mean as to be capable of accepting from any one a favor of which i could be supposed that I was unworthy

These considerations taken together were the reason why, for the las three years, I have been unwilling to publish the treatise I had o hand, and why I even resolved to give publicity during my life to n other that was so general, or by which the principles of my physic might be understood. But since then, two other reasons have come int operation that have determined me here to subjoin some particula specimens, and give the public some account of my doings and designs Of these considerations, the first is, that if I failed to do so, man who were cognizant of my previous intention to publish some writings might have imagined that the reasons which induced me to refrain fro so doing, were less to my credit than they really are; for although am not immoderately desirous of glory, or even, if I may venture so t say, although I am averse from it in so far as I deem it hostile t repose which I hold in greater account than aught else, yet, at th same time, I have never sought to conceal my actions as if they wer crimes, nor made use of many precautions that I might remain unknown and this partly because I should have thought such a course of conduc a wrong against myself, and partly because it would have occasioned m some sort of uneasiness which would again have been contrary to th perfect mental tranquillity which I court. And forasmuch as, whil thus indifferent to the thought alike of fame or of forgetfulness, have yet been unable to prevent myself from acquiring some sort o reputation, I have thought it incumbent on me to do my best to sav myself at least from being ill-spoken of. The other reason that ha determined me to commit to writing these specimens of philosophy is that I am becoming daily more and more alive to the delay which m design of self-instruction suffers, for want of the infinity o experiments I require, and which it is impossible for me to mak without the assistance of others: and, without flattering myself s much as to expect the public to take a large share in my interests, am yet unwilling to be found so far wanting in the duty I owe t myself, as to give occasion to those who shall survive me to make i matter of reproach against me some day, that I might have left the many things in a much more perfect state than I have done, had I no too much neglected to make them aware of the ways in which they coul have promoted the accomplishment of my designs

And I thought that it was easy for me to select some matters whic should neither be obnoxious to much controversy, nor should compel m to expound more of my principles than I desired, and which should ye be sufficient clearly to exhibit what I can or cannot accomplish in th sciences. Whether or not I have succeeded in this it is not for me t say; and I do not wish to forestall the judgments of others by speakin myself of my writings; but it will gratify me if they be examined, and to afford the greater inducement to this I request all who may have an objections to make to them, to take the trouble of forwarding these t my publisher, who will give me notice of them, that I may endeavor t subjoin at the same time my reply; and in this way readers seeing bot at once will more easily determine where the truth lies; for I do no engage in any case to make prolix replies, but only with perfec frankness to avow my errors if I am convinced of them, or if I canno perceive them, simply to state what I think is required for defense o the matters I have written, adding thereto no explication of any ne matte that it may not be necessary to pass without end from one thin to another

If some of the matters of which I have spoken in the beginning of th "Dioptrics" and "Meteorics" should offend at first sight, because call them hypotheses and seem indifferent about giving proof of them, request a patient and attentive reading of the whole, from which I hop those hesitating will derive satisfaction; for it appears to me tha the reasonings are so mutually connected in these treatises, that, a the last are demonstrated by the first which are their causes, th first are in their turn demonstrated by the last which are thei effects. Nor must it be imagined that I here commit the fallacy whic the logicians call a circle; for since experience renders the majorit of these effects most certain, the causes from which I deduce them d not serve so much to establish their reality as to explain thei existence; but on the contrary, the reality of the causes i established by the reality of the effects. Nor have I called the hypotheses with any other end in view except that it may be known tha I think I am able to deduce them from those first truths which I hav already expounded; and yet that I have expressly determined not to d so, to prevent a certain class of minds from thence taking occasion t build some extravagant philosophy upon what they may take to be m principles, and my being blamed for it. I refer to those who imagin that they can master in a day all that another has taken twenty year to think out, as soon as he has spoken two or three words to them o the subject; or who are the more liable to error and the less capabl of perceiving truth in very proportion as they are more subtle an lively. As to the opinions which are truly and wholly mine, I offer n apology for them as new,--persuaded as I am that if their reasons b well considered they will be found to be so simple and so conformed, t common sense as to appear less extraordinary and less paradoxical tha any others which can be held on the same subjects; nor do I even boas of being the earliest discoverer of any of them, but only of havin adopted them, neither because they had nor because they had not bee held by others, but solely because reason has convinced me of thei truth

Though artisans may not be able at once to execute the invention whic is explained in the "Dioptrics," I do not think that any one on tha account is entitled to condemn it; for since address and practice ar required in order so to make and adjust the machines described by me a not to overlook the smallest particular, I should not be les astonished if they succeeded on the first attempt than if a person wer in one day to become an accomplished performer on the guitar, by merel having excellent sheets of music set up before him. And if I write i French, which is the language of my country, in preference to Latin which is that of my preceptors, it is because I expect that those wh make use of their unprejudiced natural reason will be better judges o my opinions than those who give heed to the writings of the ancient only; and as for those who unite good sense with habits of study, who alone I desire for judges, they will not, I feel assured, be so partia to Latin as to refuse to listen to my reasonings merely because expound them in the vulgar tongue

In conclusion, I am unwilling here to say anything very specific of th progress which I expect to make for the future in the sciences, or t bind myself to the public by any promise which I am not certain o being able to fulfill; but this only will I say, that I have resolve to devote what time I may still have to live to no other occupatio than that of endeavoring to acquire some knowledge of Nature, whic shall be of such a kind as to enable us therefrom to deduce rules i medicine of greater certainty than those at present in use; and that m inclination is so much opposed to all other pursuits, especially t such as cannot be useful to some without being hurtful to others, tha if, by any circumstances, I had been constrained to engage in such, do not believe that I should have been able to succeed. Of this I her make a public declaration, though well aware that it cannot serve t procure for me any consideration in the world, which, however, I do no in the least affect; and I shall always hold myself more obliged t those through whose favor I am permitted to enjoy my retirement withou interruption than to any who might offer me the highest earthl preferments