are expressible, in principle, by differential equations in
which time (in the sense in which the mathematician takes this word)
would play the role of independent variable. Is it so with the laws of
life? Does the state of a living body find its complete explanation in
the state immediately before? Yes, if it is agreed _a priori_ to liken
the living body to other bodies, and to identify it, for the sake of the
argument, with the artificial systems on which the chemist, physicist,
and astronomer operate. But in astronomy, physics, and chemistry the
proposition has a perfectly definite meaning: it signifies that certain
aspects of the present, important for science, are calculable as
functions of the immediate past. Nothing of the sort in the domain of
life. Here calculation touches, at most, certain phenomena of organic
_destruction_. Organic _creation_, on the contrary, the evolutionary
phenomena which properly constitute life, we cannot in any way subject
to a mathematical treatment. It will be said that this impotence is due
only to our ignorance. But it may equally well express the fact that the
present moment of a living body does not find its explanation in the
moment immediately before, that _all_ the past of the organism must be
added to that moment, its heredity--in fact, the whole of a very long
history. In the second of these two hypotheses, not in the first, is
really expressed the present state of the biological sciences, as well
as their direction. As for the idea that the living body might be
treated by some superhuman calculator in the same mathematical way as
our solar system, this has gradually arisen from a metaphysic which has
taken a more precise form since the physical discoveries of Galileo, but
which, as we shall show, was always the natural metaphysic of the human
mind. Its apparent clearness, our impatient desire to find it true, the
enthusiasm with which so many excellent minds accept it without
proof--all the seductions, in short, that it exercises on our thought,
should put us on our guard against it. The attraction it has for us
proves well enough that it gives satisfaction to an innate inclination.
But, as will be seen further on, the intellectual tendencies innate
to-day, which life must have created in the course of its evolution, are
not at all meant to supply us with an explanation of life: they have
something else to do.

Any attempt to distinguish between an artificial and a natural