The older doctrine which we have called universal mechanism is a theory about the nature of the universe, closely linked with the early modern version of materialism. Universal mechanism held that the universe is best understood as a completely mechanical system--that is, a system composed entirely of matter in motion under a complete and regular system of laws of nature. The mechanists understood the achievements of the Scientific Revolution to show that every phenomenon in the universe could eventually be explained in terms of mechanical laws: that is, in terms of natural laws governing the motion and collision of matter. It follows that mechanism is a form of thoroughgoing determinism: if all phenomena can be explained entirely through the motion of matter under physical laws, then just as surely as the gears of a clock completely determine that it will strike 2:00 an hour after it strikes 1:00, all phenomena are completely determined by the properties of that matter and the operations of those natural laws. (Indeed, the determinism implied by universal mechanism is even stronger than clockwork: whereas the mechanism of a clock may cease to work predictably as its parts break down, the "parts" of the system in universal mechanism are nothing less than everything in the universe — anything that they "broke down" into would still be a part of the universe, and so would still be subject to the mechanistic laws of nature. The French mechanist and determinist Pierre Simon de Laplace memorably formulated the sweeping implications of this thesis by saying:
The mechanistic worldview gained considerable favor with the revolutionary successes of Isaac Newton, whose work in mechanics seemed to successfully explain the motion of everything in heaven and in earth according to the operation of a single mechanical principle. To be sure, that principle — universal gravitation — was something of a disappointment to the older cadre of mechanists, since mechanism originally sought to explain all phenomena entirely in terms of the motion and collision of material bodies, whereas Newton's principle of gravitation required action at a distance. Nevertheless, the generation of philosophers who were inspired by Newton's example carried the mechanist banner. Chief among them were French philosophes such as Julien Offray de La Mettrie and Denis Diderot (see also: French materialism).
Universal mechanism has since fallen into disfavor - not so much because philosophers are less inclined toward a scientific worldview now than they were in the 17th and 18th centuries, but rather because physical science has abandoned the mechanistic worldview in favor of one in which phenomena such as energy are held to be at least coequal with matter as constituents of the universe, and — possibly, under some interpretations — universal determinism is denied. The motivations that led some philosophers to mechanism in the 17th and 18th centuries now lead philosophers of a similar temperament towards physicalism, which leaves the specification of the primitive contents of the universe to a "completed physics."
Today, as in the past, the main points of debate between anthropic mechanists and anti-mechanists is mainly occupied with two topics: the mind — and consciousness, in particular — and free will. Anti-mechanists argue that anthropic mechanism is incompatible with our commonsense intuitions: in philosophy of mind they argue that unconscious matter cannot completely explain the phenomenon of consciousness, and in metaphysics they argue that anthropic mechanism implies determinism about human action, which (they argue) is incompatible with our understanding of ourselves as creatures with free will. In order to hold on to the ways in which we understand ourselves, they argue, we are logically committed to rejecting mechanism. Contemporary philosophers who have argued for this position include Norman Malcolm and David Chalmers.
Anthropic mechanists typically respond in one of two ways: either they dig in or they try to find a way to finesse the problem. In the former case, they agree with anti-mechanists that mechanism conflicts with some of our commonsense intuitions, but go on to argue that our commonsense intuitions are simply mistaken and need to be revised. Down this path lie eliminative materialism in philosophy of mind, and hard determinism on the question of free will. This option is popular with some scientists, but it is rejected by most philosophers. (Although not by its most well-known advocate, the eliminative materialist Paul Churchland. The more common option, amongst philosophers who adopt anthropic mechanism, is to argue that the arguments given for incompatibility are specious: whatever it is we mean by "consciousness" and "free will," they urge, it is fully compatible with a mechanistic understanding of the human mind and will. As a result they tend to argue for one or another non-eliminativist physicalist theories of mind, and for compatibilism on the question of free will. Contemporary philosophers who have argued for this sort of account include J. J. C. Smart and Daniel Dennett.