tion of penetrating into the
mechanism of molecules, and we were no longer satisfied to look at
matter as a mass, the mechanical solutions seemed undetermined and the
stability of the edifices thus constructed was insufficiently
demonstrated.

Returning then to our starting-point, many contemporary physicists
wish to subject Descartes' idea to strict criticism. From the
philosophical point of view, they first enquire whether it is really
demonstrated that there exists nothing else in the knowable than
matter and movement. They ask themselves whether it is not habit and
tradition in particular which lead us to ascribe to mechanics the
origin of phenomena. Perhaps also a question of sense here comes in.
Our senses, which are, after all, the only windows open towards
external reality, give us a view of one side of the world only;
evidently we only know the universe by the relations which exist
between it and our organisms, and these organisms are peculiarly
sensitive to movement.

Nothing, however, proves that those acquisitions which are the most
ancient in historical order ought, in the development of science, to
remain the basis of our knowledge. Nor does any theory prove that our
perceptions are an exact indication of reality. Many reasons, on the
contrary, might be invoked which tend to compel us to see in nature
phenomena which cannot be reduced to movement.

Mechanics as ordinarily understood is the study of reversible
phenomena. If there be given to the parameter which represents
time,[1] and which has assumed increasing values during the duration
of the phenomena, decreasing values which make it go the opposite way,
the whole system will again pass through exactly the same stages as
before, and all the phenomena will unfold themselves in reversed
order. In physics, the contrary rule appears very general, and
reversibility generally does not exist. It is an ideal and limited
case, which may be sometimes approached, but can never, strictly
speaking, be met with in its entirety. No physical phenomenon ever
recommences in an identical manner if its direction be altered. It is
true that certain mathematicians warn us that a mechanics can be
devised in which reversibility would no longer be the rule, but the
bold attempts made in this direction are not wholly satisfactory.

[Footnote 1: I.e., the time-curve.--ED.]

On the other hand, it is established that if a mechanical explanation
of a phenomenon can be given,