Mechanism is the belief that natural wholes (principally living things) are like complicated machines or artifacts, composed of parts lacking any intrinsic relationship to each other. Thus, the source of an apparent thing's activities is not the whole itself, but its parts or an external influence on the parts.
The doctrine of mechanism in philosophy comes in two different flavors. 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.
There is no constant meaning in the history of philosophy for the word Mechanism. Originally, the term meant that cosmological theory which ascribes the motion and changes of the world to some external force. In this view material things are purely passive, while according to the opposite theory (i. e., Dynamism), they possess certain internal sources of energy which account for the activity of each and for its influence on the course of events; These meanings, however, soon underwent modification. The question as to whether motion is an inherent property of bodies, or has been communicated to them by some external agency, was very often ignored. With a large number of cosmologists the essential feature of Mechanism is the attempt to reduce all the qualities and activities of bodies to quantitative realities, i. e. to mass and motion. But a further modification soon followed. Living bodies, as is well known, present at first sight certain characteristic properties which have no counterpart in lifeless matter. Mechanism aims to go beyond these appearances. It seeks to explain all "vital" phenomena as physical and chemical facts; whether or not these facts are in turn reducible to mass and motion becomes a secondary question, although Mechanists are generally inclined to favour such reduction. The theory opposed to this biological mechanism is no longer Dynamism, but Vitalism or Neo-vitalism, which maintains that vital activities cannot be explained, and never will be explained, by the laws which govern lifeless matter.
The older doctrine, here called universal mechanism, is the ancient philosophies closely linked with materialism and reductionism, especially that of the atomists and to a large extent, stoic physics. They held that the universe is reducible to completely mechanical principles—that is, the motion and collision of matter. Later mechanists believed the achievements of the scientific revolution 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 thorough going determinism: if all phenomena can be explained entirely through the motion of matter under physical laws, then even more surely than the gears of a clock determine that it must strike 2:00 an hour after striking 1:00, all phenomena must be completely determined: whether past, present or future.