ense of the word, in order to discover a law which is the very
type of scientific knowledge as we understand it. What distinguishes
modern science is not that it is experimental, but that it experiments
and, more generally, works only with a view to measure.

For that reason it is right, again, to say that ancient science applied
to _concepts_, while modern science seeks _laws_--constant relations
between variable magnitudes. The concept of circularity was sufficient
to Aristotle to define the movement of the heavenly bodies. But, even
with the more accurate concept of elliptical form, Kepler did not think
he had accounted for the movement of planets. He had to get a law, that
is to say, a constant relation between the quantitative variations of
two or several elements of the planetary movement.

Yet these are only consequences--differences that follow from the
fundamental difference. It did happen to the ancients accidentally to
experiment with a view to measuring, as also to discover a law
expressing a constant relation between magnitudes. The principle of
Archimedes is a true experimental law. It takes into account three
variable magnitudes: the volume of a body, the density of the liquid in
which the body is immersed, the vertical pressure that is being exerted.
And it states indeed that one of these three terms is a function of the
other two.

The essential, original difference must therefore be sought elsewhere.
It is the same that we noticed first. The science of the ancients is
static. Either it considers in block the change that it studies, or, if
it divides the change into periods, it makes of each of these periods a
block in its turn: which amounts to saying that it takes no account of
time. But modern science has been built up around the discoveries of
Galileo and of Kepler, which immediately furnished it with a model. Now,
what do the laws of Kepler say? They lay down a relation between the
areas described by the heliocentric radius-vector of a planet and the
_time_ employed in describing them, a relation between the longer axis
of the orbit and the _time_ taken up by the course. And what was the
principle discovered by Galileo? A law which connected the space
traversed by a falling body with the _time_ occupied by the fall.
Furthermore, in what did the first of the great transformations of
geometry in modern times consist, if not in introducing--in a veiled
form, it is true--time and movement even in the