Pierre Duhem on Laws - Dictionary of Arguments
Definition Experimental Law/Duhem: Summary of experimental experiences that allow for predictions. ((s) Only predictions about future experiments, not about reality).
Instead of remembering the various cases of the refraction of light, we can reproduce or simulate all those occurring instances immediately.
Experimental physics gives us the laws as a whole group, unsegmented. The theorist classifies the variation that opposes the observer. Wherever there is order, there is also beauty.
The goal of every physical theory is the representation of experimental laws. (Experimental Law).
The physical law is not only the essence of a myriad of experiments.
Laws/Duhem: In the same way as the laws of ordinary understanding are based on the observation of facts by the natural resources of man, the laws of physics are based on the results of physical experiments.
Almost everything we have said about the experiments can be extended to the physical laws. E.g. the law of the ordinary mind: Every man is mortal. This law certainly combines abstract expressions. But these abstractions are by no means theoretical symbols.
Abstractions/Duhem: E.g. Abstraction of the ordinary mind: Before the thunder is heard, one sees the flash. The concepts are abstract, but the sensory rumbling and twitching is recognizable.
This is no longer the case with the laws of physics.
E.g. at constant temperature, the volumes occupied by the same gas mass are inversely proportional to the pressures under which it is placed. The notions are not only abstract, but symbolic to boot, and the symbols get a meaning only through the physical theories. The relations are by no means immediate; they are only produced by means of instruments.
Now there are theories which in a certain way exclude each other, or components, which are assigned in different ways depending on the theory.
E.g. Law: All gases compress and dilate in the same way. Now we ask a physicist whether the iodine vapor follows this law or not. A physicist argues that the iodine vapor is a simple gas. The density relative to the air is constant. The experiment now shows that the density of the iodine vapor relative to air depends on the temperature and the pressure. He now concludes that iodine vapor does not correspond to the given law.
According to another physicist, iodine vapor is not a simple gas, but the mixture of two gases. Then the law is no longer valid that the density is constant with respect to the air, but rather that it varies with temperature and pressure. Our second physicist now concludes that iodine vapor is no exception to the rule.
Thus the two physicists have completely different opinions as to a law which they both pronounce in the same form. They utter the same word and mean different theorems. In order to compare this expression with the reality, they carry out calculations so different that the one can find that the law is confirmed by the facts while the other considers it to be disproved.
Definition Physical Law/Duhem: a symbolic relationship whose application to concrete reality demands that one should know and accept a whole group of theories.
... It will be said that an original law was by no means overturned by the later attempts, but the experiments had merely shown that the new law must be added.
But those who say this must recognize that the primitive law must be given with special conditions, so that it does not lead to serious errors. The old law can no longer stand alone!
The physical laws are therefore all provisional, since no revisions can be ruled out for the future.
E.g. the gravitational law is violated by capillary phenomena. So that it is not disproved, one must change it. One may consider that the formula according to which the attraction is inversely proportional to the square of the distance is not an exact, but only an approximate one._____________Explanation of symbols: Roman numerals indicate the source, arabic numerals indicate the page number. The corresponding books are indicated on the right hand side. ((s)…): Comment by the sender of the contribution. Translations: Dictionary of Arguments The note [Author1]Vs[Author2] or [Author]Vs[term] is an addition from the Dictionary of Arguments. If a German edition is specified, the page numbers refer to this edition.
La théorie physique, son objet et sa structure, Paris 1906
Ziel und Struktur der physikalischen Theorien Hamburg 1998