experiment, and pushing things to extremes, took pleasure in proposing
very curious mechanical models which were often strange images of
reality. The most illustrious of them, Lord Kelvin, may be considered
as their representative type, and he has himself said: "It seems to me
that the true sense of the question, Do we or do we not understand a
particular subject in physics? is--Can we make a mechanical model
which corresponds to it? I am never satisfied so long as I have been
unable to make a mechanical model of the object. If I am able to do
so, I understand it. If I cannot make such a model, I do not
understand it." But it must be acknowledged that some of the models
thus devised have become excessively complicated, and this
complication has for a long time discouraged all but very bold minds.
In addition, when it became a question 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.