To save the phenomena
Pierre Duhem
p xvi
"physics is not capable of proving its postulates, nor does it have
to prove them"
Averroes De Caelo 2.35
"We find nothing in the mathematical sciences that would lead us to
believe that accentrics and epicycles exist.
For astronomers propose the existence of these orbits as if they
were principles and then deduce conclusions from them which are
exactly what the senses can ascertain. In no way do they demostrate
by such results that the assumptions they have employed as
principles are, conversly, necessities.
Now, through logic we know that every demonstration goes from what is better known to what is more obscure. If what is better known is posterior to what is less known, we have a "demonstration quia." But if what is better known precedes what is less known, two situations may arise: It may be that the existance of the object of
demonstration is obscure, while its cause is known. In such a case
we have an absolute demonstration, which makes known both the
existence and the cause of its object. But if what is unknown is
the cause of the object, we shall only have a "demonstration proper
quid".
But the theory we are talking about belongs to neither of these
modes of demonstration. For in this theory the principles are
hidden from us, but they are in no way necessitated by the known
effects. Astronomers are satisfied to assume such principles,
although they do not know them.
Furthermore, if you consider the effects that astronomers bear in
mind when they advance their principles, you will find nothing in
them from which it follows essentially and with necessity that this
is the way things are. Having laid down unknown principles and
having drived known conclusions from them, the astronomers have
merely assumed the truth of the conversion.
The astronomer must, therefore, construct an astronomical system
such that the celestial motions are yielded by it and that
nothing that is from the standpoint of physics impossible is
implied...Ptolemy was unable to see astronomy on its true
foundations....The epicycle and accentric are impossible.
We must, therefore, apply ourselves to a new investigation
concerning that genuine astronomy whose foundations are
principles of physics...Actually, in our times astronomy
is nonexistent; what we have is something that fits
calculation but does not agree with what is.
When I was young, I hoped to be able to complete this
investigation myself; now that I am old, I have abandoned
that hope; but perhaps these words will induce someone else
to undertake such a study.
(and al-Bitrogi his student fulfilled this wish)
Metaphysica, Ibn Rushd 4, comm. 45
To propose mathematical hypotheses a priori, and then to derive
conclusions from them that are faithful representations of the
facts of observation is the essential task of anyone who constructs
a theory. It would be quite absurd to think that experience, when
it bears out the results of the deductions, transforms their
premises into demonstrated truths. Nothing goes to show that
altogether different premises might not have led to the same
conclusions.
P.Duhem
No matter how numerous and exact the confirmations that are brought to a theory by experience, the hypotheses supporting the theory never attain to the certainty of commonsense truths. It would be a serious mistake to think that they do. To defend such a position in our day, after history has witnessed the collapse of so many
theories long accepted without dispute, one would have to be even
more naive.
p.66
For the astronomer's job consists of the following: To gather
together the history of the celestial movements by means of
painstakingly and skilfully made observations, and then-since he
cannot by any line of reasoning reach the true causes of these
movements-to think up or construct whatever hypotheses he pleases
such that, on their assumption, the selfsame movements, past and
future both, can be calculated by means of the principles of
geometry...It is not necessary that these hypotheses be true. They
need not even be likely. This one thing suffices that the calculation to which they lead agree with the result of observation.
..Obviously this science simply does not know the causes of the
irregularity of apparent movements. It thinks up fictive causes
which generally speaking, it views as known with certainty; yet it
is not with an eye to ever persuading anyone that this is how
things really are that it so conceives of the hypotheses, but
solely to set up correct computation. Sometimes alternative
hypotheses are available with which to account for one and the same
movement; the accentric and epicycle in the theory of solar motion
are a case in point. In such a case, the astronomer will by
preference choose the hypothesis that is easier to grasp while the
philosopher tends to seek out likelihood. Neither the one nor the
other can, however, either conceive or enunciate the least
certainty, unless he be the recipient of some divine revelation...Let no one, then expect from astronomy any doctrine about these hypotheses that is certain. Astronomy can give him nothing of the sort. Let him take care not to take as true, assumptions which were fabricated for quite a different purpose, lest, far from gaining access to astronomical science, he be turned away from it, and leave it more stupid than he was before.
p.68
On April 20, 1541, Osiander writes to Copernicus:
As for hypotheses, this is what I have always thought on that
subject: they are not articles of faith, they are merely the basis
of calculation; even if they should be false, that hardly matters,
so long as they reproduce the...of movements exactly.
p.69
In 1541, Gemma Frisius, the celebrated Dutch astronomer, writes a
letter to Dantiscus, from Louvain, in which he speaks of Copernicus
in the following terms:
I won't become embroiled in any argument about the hypotheses he
uses for his demonstration; I don't investigate what they are nor
what portion of truth they conatin. Ir hardly matters to me whether
he claims that the earth moves or declares it immobile, so long as
we get an absolutely exact knowledge of the movements of the stars
and of the periods of their movements and so long as both are
reduced to altogether exact calculations.
p.109
Galileo writes:
The quickest and surest way to show that the position of Copernicus is not contrary to Scripture is, as I see it, to show by a thousand proofs that this proposition is true and that the contrary position cannot be maintained at all. Consequently, since two truths cannot contradict each other, the position recognozed as true necessarily agrees with Holy Scripture.
Galileo's notions of the validity of experimental method and the
art of using it are nearly those that Bacon was later to formulate.
Galileo conceives of the proof of a hypothesis in imitation of the
_reductio ad absurdum_ proofs that are used in geometry. Experience, by convicting one system of error, confers certainty on its opposite. Experimental science advances by a series of dilemmas, each resolved an _experimentatum crucis_.
Since this manner of conceiving of the method of experiment was so
simple, it was bound to become extremely fashinable; but because it
was too simple, it was entirely in error. Granted that the phenomena
are no longer saved by Ptolemy's system; the falsity of that system
must then be acknowledged. But from this it does not by any means
follow that the system of Copernicus is true; the latter is after
all, not purely and simply the contradictory of the Ptolemaic system.
..no matter how numerous and exact the confirmations by experience, they can never transform a hypothesis into certain truth, for this would require, in addition, demonstration of the proposition that these same experiential facts would flagrantly contradict any
other hypotheses that might be conceived.
Structure of physical theory.
p.187
..the physicist can never subject an isolated hypothesis to
experimental test, but only a whole group of hypotheses; when
the experiment is in disagreement with his predictions, what
he learns is that at least one of the hypotheses constituting
this group is unacceptable and ought to be modified; but the
experiment does not designate which one should be changed.
.....
Physical science is a system that must be taken as a whole;
it is an organism in which one part cannot be made to
function except when the parts that are most remote from it
are called into play, some more so than others, but all to
some degree. If something goes wrong, if some dicomfort is
felt in the functioning of the organism, the physicist will
have to ferret out through its effect on the entire system
which organ needs to remedied or modified without the
possibility of isolating this organ and examining it apart.
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