Mechanism is the belief that natural wholes (principally living things) are similar to complicated machines or artifacts, composed of parts lacking any intrinsic relationship to each other.

The doctrine of mechanism in philosophy comes in two different varieties. They are both doctrines of metaphysics, but they are different in scope and ambitions: the first is a global doctrine about nature; the second is a local doctrine about humans and their minds, which is hotly contested. For clarity, we might distinguish these two doctrines as universal mechanism and anthropic mechanism.

Mechanical philosophy is a form of natural philosophy which compares the universe to a large-scale mechanism (i.e. a machine). Mechanical philosophy is associated with the Scientific Revolution of early modern Europe. One of the first expositions of universal mechanism is found in the opening passages of Leviathan by Thomas Hobbes, published in 1651.

Some intellectual historians and critical theorists argue that early mechanical philosophy was tied to disenchantment and the rejection of the idea of nature as living or animated by spirits or angels. Other scholars, however, have noted that early mechanical philosophers nevertheless believed in magic, Christianity and spiritualism.

Some ancient philosophies held that the universe is reducible to completely mechanical principles—that is, the motion and collision of matter. This view was closely linked with materialism and reductionism, especially that of the atomists and to a large extent, stoic physics. Later mechanists believed the achievements of the scientific revolution of the 17th century had shown that all phenomena could eventually be explained in terms of "mechanical laws": natural laws governing the motion and collision of matter that imply a determinism. If all phenomena can be explained entirely through the motion of matter under physical laws, as the gears of a clock determine that it must strike 2:00 an hour after striking 1:00, all phenomena must be completely determined, past, present or future.

The natural philosophers concerned with developing the mechanical philosophy were largely a French group, together with some of their personal connections. They included Pierre Gassendi, Marin Mersenne and René Descartes. Also involved were the English thinkers Sir Kenelm Digby, Thomas Hobbes and Walter Charleton; and the Dutch natural philosopher Isaac Beeckman.

Robert Boyle used "mechanical philosophers" to refer both to those with a theory of "corpuscles" or atoms of matter, such as Gassendi and Descartes, and those who did without such a theory. One common factor was the clockwork universe view. His meaning would be problematic in the cases of Hobbes and Galileo Galilei; it would include Nicolas Lemery and Christiaan Huygens, as well as himself. Newton would be a transitional figure. Contemporary usage of "mechanical philosophy" dates back to 1952 and Marie Boas Hall.

In France the mechanical philosophy spread mostly through private academies and salons; in England in the Royal Society. In England it did not have a large initial impact in universities, which were somewhat more receptive in France, the Netherlands and Germany.