If everything in chemistry is explained in a satisfactory manner without the help of phlogiston, it is by that reason alone infinitely probable that the principle does not exist; that it is a hypothetical body, a gratuitous supposition; indeed, it is in the principles of good logic, not to multiply bodies without necessity.
Interpretation
The quote suggests that if something can be explained without a certain concept, that concept is likely unnecessary or non-existent.
Antoine Lavoisier’s quote reflects the philosophical principle known as Occam's Razor, which posits that the simplest explanation requiring the least number of assumptions is usually the correct one. In the context of chemistry, Lavoisier argues against the existence of phlogiston, a now-discredited theory, by stating that if a scientific phenomenon can be adequately explained without invoking it, then its existence is likely unwarranted and unnecessary for understanding the principles at play.
In practice
During a science lecture discussing historical theories, this quote can illustrate the evolution of scientific thought.
Imagination, on the contrary, which is ever wandering beyond the bounds of truth, joined to self-love and that self-confidence we are so apt to indulge, prompt us to draw conclusions which are not immediately derived from facts.
We think only through the medium of words. Languages are true analytical methods. Algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method. The art of reasoning is nothing more than a language well arranged.
We must trust to nothing but facts: These are presented to us by Nature, and cannot deceive. We ought, in every instance, to submit our reasoning to the test of experiment, and never to search for truth but by the natural road of experiment and observation.
Perhaps... some day the precision of the data will be brought so far that the mathematician will be able to calculate at his desk the outcome of any chemical combination, in the same way, so to speak, as he calculates the motions of celestial bodies.
It took them only an instant to cut of that head, but it is unlikely that a hundred years will suffice to reproduce a singular one.
While I thought myself employed only in forming a nomenclature, and while I proposed to myself nothing more than to improve the chemical language, my work transformed itself by degrees, without my being able to prevent it, into a treatise upon the Elements of Chemistry.
There must be no barriers for freedom of inquiry... There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors.
No new creation or destruction of matter is within the reach of chemical agency. We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen.
The time has come to recognize that food, how we produce it, process it, package it, sell it, cook it and eat it, is as important as any other issue.
If you represent the Earth's lifetime by a single year, say from January when it was made to December, the 21st-century would be a quarter of a second in June - a tiny fraction of the year. But even in this concertinaed cosmic perspective, our century is very, very special: the first when humans can change themselves and their home planet.
In more than one respect, the exploring of the Solar System and homesteading other worlds constitutes the beginning, much more than the end, of history.
The feeling of awed wonder that science can give us is one of the highest experiences of which the human psyche is capable. It is a deep aesthetic passion to rank with the finest that music and poetry can deliver. It is truly one of the things that make life worth living and it does so, if anything, more effectively if it convinces us that the time we have for living is quite finite.
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