Dans les champs de l'observation le hasard ne favorise que les esprits préparés.
In the field of observation, chance favours only the prepared mind.

Il n'existe pas de sciences appliquées, mais seulement des applications de la science.
There are no such things as applied sciences, only applications of science.

After death, life reappears in a different form and with different laws. It is inscribed in the laws of the permanence of life on the surface of the earth and everything that has been a plant and an animal will be destroyed and transformed into a gaseous, volatile and mineral substance.

Analogy cannot serve as proof.

Are the atoms of the dextroacid (tartaric) grouped in the spirals of a right-hand helix or situated at the angles of an irregular tetrahedron, or arranged in such or such particular unsymmetrical fashion? We are unable to reply to these questions. But there can be no reason for doubting that the grouping of the atoms has an unsymmetrical arrangement with a non-superimposable image. It is not less certain that the atoms of the laevo-acid realize precisely an unsymmetrical arrangement of the inverse of the above.

As I show you this liquid, I too could tell you, 'I took my drop of water from the immensity of creation, and I took it filled with that fecund jelly, that is, to use the language of science, full of the elements needed for the development of lower creatures. And then I waited, and I observed, and I asked questions of it, and I asked it to repeat the original act of creation for me; what a sight it would be! But it is silent! It has been silent for several years, ever since I began these experiments. Yes! And it is because I have kept away from it, and am keeping away from it to this moment, the only thing that it has not been given to man to produce, I have kept away from it the germs that are floating in the air, I have kept away from it life, for life is the germ, and the germ is life.'

As in the experimental sciences, truth cannot be distinguished from error as long as firm principles have not been established through the rigorous observation of facts.

Every chemical substance, whether natural or artificial, falls into one of two major categories, according to the spatial characteristic of its form. The distinction is between those substances that have a plane of symmetry and those that do not. The former belong to the mineral, the latter to the living world.

Herrmann Pidoux and Armand Trousseau stated 'Disease exists within us, because of us, and through us', Pasteur did not entirely disagree, 'This is true for certain diseases', he wrote cautiously, only to add immediately: 'I do not think that it is true for all of them'.

I give them experiments and they respond with speeches.

I propose to provide proof... that just as always an alcoholic ferment, the yeast of beer, is found where sugar is converted into alcohol and carbonic acid, so always a special ferment, a lactic yeast, is found where sugar is transformed into lactic acid. And, furthermore, when any plastic nitrogenated substance is able to transform sugar into that acid, the reason is that it is a suitable nutrient for the growth of the [lactic] ferment.

If it is a terrifying thought that life is at the mercy of the multiplication of these minute bodies [microbes], it is a consoling hope that Science will not always remain powerless before such enemies...

If science has no country, the scientist should have one, and ascribe to it the influence which his works may have in this world.

In that memorable year, 1822: Oersted, a Danish physicist, held in his hands a piece of copper wire, joined by its extremities to the two poles of a Volta pile. On his table was a magnetized needle on its pivot, and he suddenly saw (by chance you will say, but chance only favours the mind which is prepared) the needle move and take up a position quite different from the one assigned to it by terrestrial magnetism. A wire carrying an electric current deviates a magnetized needle from its position. That, gentlemen, was the birth of the modern telegraph.

Let me tell you the secret that has led me to my goal. My only strength lies in my tenacity.

Live in the serene peace of laboratories and libraries

My present and most fixed opinion regarding the nature of alcoholic fermentation is this: The chemical act of fermentation is essentially a phenomenon correlative with a vital act, beginning and ending with the latter. I believe that there is never any alcoholic fermentation without their being simultaneously the organization, development, multiplication of the globules, or the pursued, continued life of globules which are already formed.

Nothing is lost and nothing is created in the operations of art as those of nature.

Science knows no country because knowledge belongs to humanity, and is the torch which illuminates the world. Science is the highest personification of the nation because that nation will remain the first which carries the furthest the works of thought and intelligence.

The artificial products do not have any molecular dissymmetry; and I could not indicate the existence of a more profound separation between the products born under the influence of life and all the others.

The only thing that can bring joy is work.

The universe is an asymmetrical entity. I am inclined to believe that life as it is manifested to us must be a function of the asymmetry of the universe or of the consequence of this fact. The universe is asymmetrical; for if one placed the entire set of bodies that compose the solar system, each moving in its own way, before a mirror, the image shown would not be superimposable on the reality.

The universe is asymmetric and I am persuaded that life, as it is known to us, is a direct result of the asymmetry of the universe or of its indirect consequences. The universe is asymmetric.
Acknowledging the role of molecules that have stereoisomers, some the mirror image of the others, and microorganisms whose chemistry prefers only one of those forms.

There is no such thing as a special category of science called applied science; there is science and its applications, which are related to one another as the fruit is related to the tree that has borne it.

These microscopic organisms form an entire world composed of species, families and varieties whose history, which has barely begun to be written, is already fertile in prospects and findings of the highest importance. The names of these organisms are very numerous and will have to be defined and in part discarded. The word microbe which has the advantage of being shorter and carrying a more general meaning, and of having been approved by my illustrious friend, M. Littré, the most competent linguist in France, is one we will adopt.

To demonstrate experimentally that a microscopic organism actually is the cause of a disease and the agent of contagion, I know no other way, in the present state of Science, than to subject the microbe (the new and happy term introduced by M. Sédillot) to the method of cultivation out of the body.

We affirm the neutrality of Science ... Science is of no country. ... But if Science has no country, the scientist must keep in mind all that may work towards the glory of his country. In every great scientist will be found a great patriot.

Wine is the most healthful and most hygienic of beverages.

Without theory, practice is but routine born of habit. Theory alone can bring forth and develop the spirit of invention. ... [Do not] share the opinion of those narrow minds who disdain everything in science which has not an immediate application. ... A theoretical discovery has but the merit of its existence: it awakens hope, and that is all. But let it be cultivated, let it grow, and you will see what it will become.

Worship the spirit of criticism. If reduced to itself it is not an awakener of ideas or a stimulant to great things, but, without it, everything is fallible; it always has the last word.

You bring me the deepest joy that can be felt by a man whose invincible belief is that Science and Peace will triumph over Ignorance and War, that nations will unite, not to destroy, but to build, and that the future will belong to those who will have done most for suffering humanity.

We may regard the cell quite apart from its familiar morphological aspects, and contemplate its constitution from the purely chemical standpoint. We are obliged to adopt the view, that the protoplasm is equipped with certain atomic groups, whose function especially consists in fixing to themselves food-stuffs, of importance to the cell-life. Adopting the nomenclature of organic chemistry, these groups may be designated side-chains. We may assume that the protoplasm consists of a special executive centre (Leistungs-centrum) in connection with which are nutritive side-chains... The relationship of the corresponding groups, i.e., those of the food-stuff, and those of the cell, must be specific. They must be adapted to one another, as, e.g., male and female screw (Pasteur), or as lock and key (E. Fischer).

I have no patience with attempts to identify science with measurement, which is but one of its tools, or with any definition of the scientist which would exclude a Darwin, a Pasteur or a Kekulé. The scientist is a practical man and his are practical aims. He does not seek the ultimate but the proximate. He does not speak of the last analysis but rather of the next approximation. His are not those beautiful structures so delicately designed that a single flaw may cause the collapse of the whole. The scientist builds slowly and with a gross but solid kind of masonry. If dissatisfied with any of his work, even if it be near the very foundations, he can replace that part without damage to the remainder. On the whole, he is satisfied with his work, for while science may never be wholly right it certainly is never wholly wrong; and it seems to be improving from decade to decade.

But when it has been shown by the researches of Pasteur that the septic property of the atmosphere depended not on the oxygen, or any gaseous constituent, but on minute organisms suspended in it, which owed their energy to their vitality, it occurred to me that decomposition in the injured part might be avoided without excluding the air, by applying as a dressing some material capable of destroying the life of the floating particles. Upon this principle I have based a practice.

HEART, n. An automatic, muscular blood- pump. Figuratively, this useful organ is said to be the seat of emotions and sentiments—a very pretty fancy which, however, is nothing but a survival of a once universal belief. It is now known that the sentiments and emotions reside in the stomach, being evolved from food by chemical action of the gastric fluid. The exact process by which a beefsteak becomes a feeling—tender or not, according to the age of the animal from which it was cut; the successive stages of elaboration through which a caviar sandwich is transmuted to a quaint fancy and reappears as a pungent epigram; the marvelous functional methods of converting a hard-boiled egg into religious contrition, or a cream-puff into a sigh of sensibility—these things have been patiently ascertained by M. Pasteur, and by him expounded with convincing lucidity. 

A body of work such as Pasteur's is inconceivable in our time: no man would be given a chance to create a whole science. Nowadays a path is scarcely opened up when the crowd begins to pour in.

If Louis Pasteur were to come out of his grave because he heard that the cure for cancer still had not been found, NIH would tell him, “Of course we'll give you assistance. Now write up exactly what you will be doing during the three years of your grant.” Pasteur would say, “Thank you very much,” and would go back to his grave. Why? Because research means going into the unknown. If you know what you are going to do in science, then you are stupid! This is like telling Michelangelo or Renoir that he must tell you in advance how many reds and how many blues he will buy, and exactly how he will put those colors together.