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DESCARTES, RENÉ DU PERRON (b. La Haye,
Touraine, France, 31 March 1596; d. Stockholm,
Sweden, 11 February 1650), natural philosophy, scientific
method, mathematics, optics, mechanics, physiology.
know the use of some machine and see some of its parts,
easily infer from these the manner in which others which
they have not seen are made, so, from the perceptible
effects and parts of natural bodies, I have endeavoured
to find out what are their imperceptible causes and
parts.13
This reduction made the principles of the mechanistic
model the only principles operating in nature, thus
bringing the objectives of the engineer into the search
for the nature of things and throwing the entire world
of matter open to the same form of scientific inquiry
and explanation. Research, whether into cosmology
or physiology, was reduced to the discovery and elucidation
of mechanisms. He could construct in distant
space the imaginary world of Le monde and L'homme,
and later the world of the Principles, as explicitly and
unambiguously hypothetical imitations of our actual
world, made in accordance with the known laws of
mechanics. The heuristic power of the model was
that, like any other theory advanced in anticipation
of facts, its own properties suggested new questions
to put to nature. The main issue in any historical
judgment of Descartes here is not whether his own
answers were correct but whether his questions were
fruitful. In insisting that experiment and observation
alone could show whether the model corresponded
with actuality, he introduced further precision into his
theory of demonstration.
Descartes used the word demonstrer to cover both
the explanation of the observed facts by the assumed
theory and the proof of the truth of the theory. When
challenged with the criticism that this might make the
argument circular, he replied by contrasting two kinds
of hypothesis.14 In astronomy various geometrical
devices, admittedly false in nature, were employed to
yield true conclusions only in the sense that they
“saved the appearances.” But physical theories were
proposed as true. He was persuaded of the truth of
the assumption that the material world consisted of
particles in motion by the number of different effects
he could deduce, as diverse as the operation of vision,
the properties of salt, the formation of snow, the
rainbow, and so on. Thus he made range of application
the empirical criterion of truth. He wrote in
the Discours:
If some of the matters of which I have spoken in the
beginning of the Dioptrique and the Météores
should, at
first sight, shock people because I have called them
suppositions, and do not seem to bother about their
proof, let them have the patience to read them carefully
right through, and I hope that they will find themselves
satisfied. For it seems to me that the reasonings are so
interwoven that as the later ones are demonstrated by
the earlier which are their causes, these earlier ones are
reciprocally demonstrated by the later which are their
effects. And it must not be thought that in this I commit
the fallacy which logicians call arguing in a circle, for,
since experience renders the majority of these effects
very certain, the causes from which I deduce them do
not so much serve to prove them as to explain them;
on the other hand, the causes are proved by the effects.15
The test implied precisely by the criterion of range
of confirmation was the experimentum crucis. This is
the most obvious feature in common between Descartes's
logic of experiment and that of Francis Bacon.
Descartes described its function in the Discours:
Reviewing in my mind all the objects that have ever
been presented to my senses, I venture truly to say that
I have not there observed anything that I could not
satisfactorily explain by the principles I had discovered.
But I must also confess that the power of nature is so
ample and so vast and that these principles are so simple
and so general, that I have observed hardly any particular
effect that I could not at once recognize to be deducible
from them in several different ways, and that
my greatest difficulty is usually to discover in which of
these ways it depends on them. In such a case, I know
no other expedient than to look again for experiments
[expériences] such that their result is not the same if it
has to be explained in one of these ways as it would
be if explained in the other.16
It was a logician rather than an experimenter who
seems to have been uppermost in Descartes's application
of this criterion in the same way to very general
assumptions, such as the corpuscularian natural philosophy,
and to questions as particular as whether the
blood left the heart in systole or in diastole. Descartes
argued in La description du corps humain (1648-1649)
that whereas Harvey's theory that the blood was
forced out of the heart by a muscular contraction
might agree with the facts observed so far, “that does
not exclude the possibility that all the same effects
might follow from another cause, namely from
the dilatation of the blood which I have described.
But in order to be able to decide which of these two
causes is true, we must consider other observations
which cannot agree with both of them.”17 Harvey
replied in his Second Disquisition to Jean Riolan.
As the great optimist of the scientific movement of
the seventeenth century, Descartes habitually wrote
as if he had succeeded in discovering the true principles
of nature to such an extent that the whole
scientific program was within sight of completion.
Then, as Seth Ward neatly put it, “when the operations
of nature shall be followed up to their staticall
(and mechanicall) causes, the use of induction will
cease, and syllogisme succeed in place of it.” But
Descartes would surely have agreed with Ward's