Observation is the attention of the soul focused on objects offered by nature. An experiment is the result of this same attention directed toward phenomena produced by the labors of man. We must, therefore, include within the meaning of the generic noun observation the examination of all natural effects, not only of those that present themselves at once and without intermediary to our sight but also those we would not be able to discover without the hand of a worker, provided that this hand has not changed, altered, or disfigured them. The work necessary to reach a mine does not prevent the examination that is made of the metal's distribution, position, quantity, and color from being a simple observation. It is also by observation that we know the interior geography, that we estimate the number, position, and nature of the layers of earth, although we are obliged to resort to instruments for the excavation that allows us to see the mine. We must not consider as an experiment the opening of cadavers, the dissection of plants or animals, and certain analyses or mechanical sorting of mineral matter that scientists are obliged to do in order to be able to observe the parts that enter into their composition. The telescope of astronomers, the magnifying glass of the naturalist, and the microscope of the physicist do not prevent the knowledge acquired by these means from being the exact product of observation . All these preparations, these instruments only serve to render the different objects of observation more concrete, to remove the obstacles that prevent us from perceiving them, or to pierce the veil that hides them. But no change results from this, and there is not the slightest alteration in the nature of the observed object. It appears, nevertheless, such as it is; and this is the main difference between an observation and an experiment which decomposes, combines, and thereby gives use to rather different phenomena from those which nature presents. Thus, for example, if after a mine has been opened a chemist takes a piece of metal and throws it into some liquid solution that can dissolve it, the artificial union of these two bodies, an indispensable effect of the dissolution, will form a new compound, produce new phenomena, and, strictly speaking, make an experiment in which arbitrary results will have been substituted for natural ones. If a physiologist mixes some liquid with blood recently drawn from a living animal, he will then be making an experiment; and the knowledge that could be derived from it about the nature of blood and the alterations produced by this liquid will no longer be the fruit of a simple observation. We shall note in passing that the knowledge acquired this way is rather mediocre and imperfect, not to say absolutely worthless. And the inferences that can be drawn about the power of remedies are very faulty, usually contradicted by observation. In general only a small advantage is now gained from experiments that investigate animals and vegetables, even from chemical experiments, which, of all experiments, are unquestionably the most trustworthy and luminous. The part of chemistry that deals with organic matter is not very rich in properly verified facts and rather far from the perfection attained in mineralogy. It is not likely that we shall be able to arrive at this point except by the discovery of the laws and the details of the mechanism of organization, a precious and fecund discovery that we must expect only from observation. Experiments on brute, inanimate bodies are much more useful and satisfying: this part of chemistry has been pushed rather far. Chemistry has succeeded in decomposing and recomposing these bodies, either by the reunion of separate elements or with elements drawn entirely from other bodies, as in artificial lighting, or in part as it is usually done in regard to reconstituted metal, by adding any phlogiston to a specific metallic loam.

Observation is the primary foundation of all the sciences, the most reliable way to arrive at one's goal, the principal means of extending the periphery of scientific knowledge and of illuminating all its points. The facts, whatever they are, constitute the true wealth of the philosopher and the subject of observation: the historian collects them, the theoretical physicist combines them, and the experimenter verifies the results of their synthesis. Several facts taken separately appear dry, sterile, and unfruitful. The moment we compare them, they acquire a certain power, assume a vitality that everywhere results from the mutual harmony, from the reciprocal support, and from a chain that binds them together. The connection of these facts and the general cause that links them together are some of the objects of reasoning, theories, and systems, while the facts are the materials. The moment a certain number of them have been gathered, some people hasten to construct; and the building is the more solid as the materials are more numerous and each one of them finds a more appropriate place. It sometimes happens that the imagination of the architect compensates for the deficiency found in the number and relation of the materials, and he manages to make them serve his plans, however defective they may be. This is the case of those audacious and eloquent theorists who, devoid of the necessary patience to observe, are content to have gathered a few facts, immediately tie them together by some ingenious system, and render their opinions plausible and attractive by the richness of their stylistic devices, the variety and force of some colorful writing, and striking and sublime images with which they know how to present their ideas. How can one resist admiring and almost believing Epicurus, Lucretius, Aristotle, Plato, and M. de Buffon? But when one is in too much of a hurry (a common shortcoming) to create a chain of all the facts that have been collected by observation, one constantly runs the risk of encountering some facts that cannot fit, that force a change in the system or destroy it entirely. And as the field of discovery is extremely vast, and its limits extend further as knowledge increases, it appears impossible to establish a general system that is always true. We should not be astonished to see some of the great men of antiquity attached to opinions that we find ridiculous, because there is every reason to presume that in times past those opinions took full account of all the observations already made and were in complete agreement with them. If we could live several centuries from now, we would see our principal systems, which appear the most ingenious and the most reliable, destroyed, scorned, and replaced by others that would then be subject to the same vicissitudes.

Observation has produced the history or the science of the facts that concern God, man, and nature. Observation of the works of God, of miracles, of religions, etc. has created Sacred History. Observation of life, deeds, customs, and mankind has given rise to Secular History. Observation of nature, of the movement of heavenly bodies, of the vicissitudes of seasons, meteors, elements, animals, vegetables, and minerals, of the mistakes of nature, of its use, of arts and crafts have all furnished the materials for the different branches of Natural History. See these words .

Observation and experiment [1] are the only paths that we have to knowledge, if one recognizes the truth of the axiom: we have nothing in our minds that did not originally come from the senses.  [2] At least these are the only means by which one can attain knowledge of objects that fall within the province of the senses. It is only through them that we can cultivate physics, and it is not doubtful that observation even in the physics of inorganic bodies infinitely surpasses experimentation in certainty and usefulness, although inanimate bodies, without life and almost without action, only offer the observer a certain number of phenomena that are relatively uniform and in appearance easy to recognize and combine. Although one cannot overlook the fact that experiments, especially those in chemistry, have shed a great light on that science, we see that the parts of physics, which are entirely within the province of observation , are the best known and perfected. It is by observation that the laws of motion have been determined, that the general properties of bodies have been known. It is to observation that we owe the discovery of gravity, attraction, acceleration of heavy bodies, and the system of Newton; on the other hand, Descartes' system was based on experience. Finally it is observation that created astronomy and carried it to this point of perfection in which we see it today, and which is such that it surpasses in certainty all the other sciences. The immense distance of the heavenly bodies, which prevented all experimentation, seemed bound to be an obstacle to our knowledge. But observation , to which astronomy was totally delivered, cleared this and all other obstacles. One can also say that celestial physics is the fruit and triumph of observation . In chemistry observation opened a vast field of experimentation: it threw light on the nature of air, water, fire, on fermentation, on the decomposition and spontaneous degeneration of bodies. It is observation that furnished almost all the materials of the excellent treatise on fire that Boerhaave collected from various physicists. There is one part of mineralogy that cannot be illuminated by the torch of observation : that is the formation and decomposition of metals in mines. And if ever we succeed in discovering the philosopher's stone, it will only be when we have seen how nature develops the metals to their different points of evolution, [3] forming each metal separately. Then the rival art that imitates nature will perhaps be able to hasten and bring about the perfect development that, according to the rather likely idea of adepts, [4] will make gold. . . .

Man ultimately, from whatever angle one considers him, is the least appropriate subject for experimentation . He is the most suitable, lofty, and interesting object of observation ; and it is only by means of this method that a certain amount of progress can be made in the sciences related to man. Experimentation here is often more than useless. Man can be considered from two principal points of view: either in regard to the moral or in connection with the physical. The observations made on moral man are, or should be, the basis of secular history, morality, and all the sciences which are derived from it. See Morality. The history of the rise and decline of the Roman empire and the immortal book on the spirit of the laws, [5] both excellent treatises on morality, are almost nothing more than an immense collection of observations made with a great deal of genius, choice, and sagacity, which furnished the illustrious author with reflections that are all the more just as they are more natural. Observations made on man considered from the physical aspect comprise the noble and divine science called medicine , which is concerned with the knowledge of man, health, illness, and the means to relieve and prevent one and to conserve the other. As this science is more important than any other one, and as it owes a great deal more to observation , and as it concerns us personally, we shall now go into a few details.

Observation was the cradle and the school of medicine in the most remote centuries of civilization when necessity became the mother of invention and disease compelled men to resort to remedies. This was before some individuals sacrificed their tranquility, their health, and their lives to the public interest by devoting themselves to a tedious, difficult, respectable science that quite often received little respect. Medicine was in the hands of everyone. The sick were exposed at the door of their homes, in the streets, or in the temples. Each passerby came to examine them and proposed the remedies that he had seen to be successful on a similar occasion or that he judged to be so. The priests made a point of copying these prescriptions, of taking notes on the remedy and the illness, if the. result was favorable. The observation of bad results would have been advantageous, and in some places they wrote these observations on the columns of the temples. In others they made all sorts of compilations that were consulted after they increased to considerable length. From these beginnings was born empiricism, whose success at first appeared so astonishing that people deified the doctors who had devoted themselves to it. All their observations are lost, and we must miss them even more, as they would surely be simple, devoid of all theoretical ideas, of all systematization, and consequently would conform more to the truth. The medicine preserved in the family of the Asclepiades and handed down from father to son was without a doubt nothing else than this interesting compilation. The first schools of medicine did not have any other books, and the Cnidian aphorisms, according to Hippocrates, were nothing but similar collections of observations . Such was the state of clinical medicine until the memorable time of this divine legislator. Some philosophers after Pythagoras had tried to add intellectual reasoning, combining the dogmas of the prevailing physical systems to the practice of medicine . They had become theoreticians, but they were doctors only in their offices. They did not see any patients. The empiricists alone who had founded medicine practiced it. Observation was their sole guide: servile but blind imitators, they often incurred the risk of confusing very different diseases, for they did not have at their disposal very precise descriptions and they were by no means aware of the value of genuine and characteristic symptoms. Empiricism was therefore necessary but insufficient. Medicine can absolutely not exist without it, but empiricism is not capable by itself of creating medicine. The great and immortal Hippocrates collected the observations of his predecessors. He even appears to have been almost solely engaged in observing himself. And he pushed the art of observation so far that he managed to change the state of medicine and raise it to a degree of perfection that no one for more than twenty centuries has yet been able to attain. Although he possessed much theoretical knowledge, the descriptions he gave of diseases were not affected by this, for they were purely empirical. These observations are simple and exact, stripped of all extraneous embellishment. They only contain facts and some rather interesting facts. He relates the observations with much detail in his books on epidemics , his aphorisms, and his prenotions concerning waters. The prorrhétiques [predictions] and the books on prognostics suppose an immense quantity of observations and are a kind of precious extract of them. What degree of certainty would not medicine have attained if all the doctors who followed him had tread in his footsteps? If each one had applied himself to observing and transmitting to us his observations with the simplicity and candor of Hippocrates, what an immense collection of facts would we now not possess? What wealth for a doctor? What an advantage for humanity? But, let us admit it, the medicine of today, and even more the medicine of the past century, despite the anatomical discoveries, the augmentation of materia medica, and the light of physics, is rather far from the perfection that was given to it by only one man. The reason for this is rather apparent: instead of observing, men spent their time reasoning, preferring the brilliant title of theoretician to the laborious and obscure occupation of observer. The errors of physics have at all times infected medicine. Theorizomania prevailed, and the more people indulged in it, the less they cultivated observation . Certain theories vicious in principle were even more so in their consequences. Asclepiades, rash and presumptuous doctor, publicly condemned the procedure of observation followed by Hippocrates, and he had some disciples. There was also formed at the same time a new sect of systematic empiricists, but the inadequacy of their method caused them to disappear rather quickly. A long time afterward appeared Galen, the famous commentator of Hippocrates, who observed a great deal but reasoned too much. He raised medicine to the pitch of philosophy. The Greeks pursued this mistake and neglected observation . They indulged themselves in hypotheses and were imitated in this respect by the Arabs, who almost entirely disfigured medicine. We have received from them only a few observations on surgery and a very exact description of smallpox found in Rhazes, Medicine passed from the hands of the ignorant Galenists, who were slavishly attached to the decisions of their master, into those of chemists, practicing doctors, filled with imagination that the steam of their furnaces still heated. The principles of their medicine were totally opposed to observation and to the study of nature. They always wanted to act and boasted about possessing dependable remedies. Their ideas were very beautiful, very specious. If they had been true, would that be desirable? The mechanists took hold of medicine and divested it of all the errors that had been introduced by chemistry, but only to substitute new ones. Observation was totally lost from sight, and they claimed to have replaced it by algebraic calculations and the application of mathematics to the human body. The so called "discovery of circulation" dazzled all minds, increased the frenzy and the craze for hypotheses, and thrust into the minds of doctors the sterile taste for experiments which were always unfruitful. Theories built on these foundations became the rules of practice, and no more was said of observation . The renewal of the sciences procured for medicine some knowledge that was irrelevant to practice, more curious than useful, more pleasant than necessary. Anatomy, for example, and natural history became the subjects of research by doctors, who were thereby diverted from observation . Clinical medicine was less cultivated and more unreliable. We only gained from this activity, moreover, a few minute details of absolutely useless value. Physiology appeared to make some progress; the knowledge of diseases and the science of symptoms were much more neglected. Therapeutics grew richer from the direction of remedies, but it was less certain about procedure and less simple about application. Recently, with Chirocism [sic] having become dominant, active medicine was brought into fashion, and with it the ill-considered use of blood-letting and purgation. Observation was pursued less than ever, because these remedies were indifferently applied in all cases; or if some observations were made, it was not difficult to perceive that men saw with preoccupied eyes and that one had to deal tactfully with certain interests when conveying the results of investigation.

Such has been the state of medicine from Hippocrates to our day, constantly passing from one sectarian to another, continually altered and obscured by hypotheses and systems that succeeded and reciprocally destroyed each other, with all the more ease, as the truth was not on any side. Plunged by the lack of observation into the greatest uncertainty, a few medical observers of small number have occasionally raised their voices, but they were stifled by the cries of theorists, or the appeal of systems prevented people from following them. See Observer. The taste for observation appears to have been recovered for some time. The writings of Sydenham, Baglivi, and Stahl have served to inspire it. The power of nature in the cure of disease, recalled to mind by the illustrious author under the improper name of soul , has contributed a good deal to the renewal of observation as a method of procedure. This system, which is vicious only because some people want to determine the quality of nature and to confuse it with the soul, is very favorable to practical medicine, provided that one does not push it to excess. He made many followers, who are to a great extent zealous supporters of observation . The philosophic spirit which happily penetrated the field of medicine tried above all to ascertain the facts, to induce men to look at everything, to examine everything, to seize the truth with ardor and to love it beyond everything else. The prodigious quantity of past errors leaves us less to be feared. Perhaps, in addition, the lights of our enlightened century together with all these considerations taken together favor the return of observation and serve to relight this torch. Medicine appears to be on the point of a great revolution. Highly esteemed systems are reduced to their right value. Several doctors properly apply themselves to observation; they follow nature and will not be long in reviving the medicine of Hippocrates, which is the true medicine of observation . Thus after much work this science may be advanced and carried to the point where it was two thousand years ago. Mankind shall still be happy if the doctors who come afterward continue to follow this path and if, always guided by the thread of observation , they avoid the errors and aberrations so shameful for themselves and so fatal to others.