As a theory, rather than a device, the view that everything happens mechanically; that is, everything can ultimately be explained in terms of certain laws of nature which apply to the behavior of matter in motion, as in the popular example of clockwork.
Ideally the laws should require as few terms as possible - perhaps just solidity, void, and motion (see also atomism - and be intuitively clear, preferably by being reducible to logical laws. For traditional atomism it is a logical law that two atoms cannot occupy the same place, so it seems that a moving atom must push another one it hits.
However, the pursuit of such an ideally simple mechanical model has proved illusory. Gravity and magnetism, at least, require pulling as well as pushing, and solidity itself has proved to be problematic.
The properties and forces appealed to by modern physics have no intuitive appeal, and intuition therefore sees no reason why they should not be added to or replaced indefinitely. However, mechanism can still take the form of an appeal to simplicity and uniformity of explanation (see also Ockham's Razor) to attack its traditional rivals such as finalism, organicism, vitalism and emergence theories with varying success.