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.
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.