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, we can find an infinity of others which
likewise account for all the peculiarities revealed by experiment.
But, as a matter of fact, no one has ever succeeded in giving an
indisputable mechanical representation of the whole physical world.