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.
I always say, 'Let your experiment speak to you.' What I mean by that is I - actually, we, or, at least, I'm not smart enough, actually, to guess how nature is working, but by looking and doing the right experiments and paying close attention to the subtleties of it, you start to catch on.
I do not want to drive across a bridge designed by an engineer who believed the numbers in structural stress models are relative truths.
It is interesting thus to follow the intellectual truths of analysis in the phenomena of nature. This correspondence, of which the system of the world will offer us numerous examples, makes one of the greatest charms attached to mathematical speculations.
We only know God in His works, but we are forced by science to admit and to believe with absolute confidence in a Directive Power-in an influence other than physical, or dynamical, or electrical forces.
Does anyone believe that the difference between the Lebesgue and Riemann integrals can have physical significance, and that whether say, an airplane would or would not fly could depend on this difference? If such were claimed, I should not care to fly in that plane.
I don't believe that the ultimate theory will come by steady work along existing lines. We need something new. We can't predict what that will be or when we will find it because if we knew that, we would have found it already!
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