Physical determinism

Physical determinism generally refers to the assertion of a deterministic physical universe (greater physical system). This holds that a complete description of the physical state of the world at any given time and a complete statement of the physical laws of nature together entail every truth as to what physical events happen after that time. In a scientific context, the term 'event' is somewhat technical, depending upon which theory one is considering, and basically is the occurrence of some 'state' peculiar to that theory, for example, a quantum state or a thermodynamic state. Physical determinism includes (but is not restricted to) nomological determinism, which holds that all future events are governed by the past or present according to all-encompassing deterministic laws.

The concept of physical determinism has also been used to denote the predictability of a physical system, although this usage is uncommon. Physical determinism can also be viewed as an observed phenomenon of our experience, or a thesis only relevant to mathematical models of physics and other physical sciences. Physical determinism has also been used as a specific deterministic hypothesis about human behavior. Although somewhat unrelated to its standard context, physical determinism has also been used in social engineering theory.

History

The notion of physical determinism takes its classical form in the ideas of Laplace, who posited (in agreement with the physics of his time) that an omniscient observer (called sometimes Laplace's demon) knowing with infinite precision the positions and velocities of every particle in the universe could predict the future entirely. Although such an omniscient observer is a hypothetical construct, and infinite precision exceeds the capacities of human measurement, the illustration is presented as a statement of what in principle would be possible if physical determinism were true, and so reduction to practice is not an issue.

Physical determinism is currently under heavy debate in modern science. For example, physical indeterminism has been proposed to accommodate various interpretations of quantum mechanics. Suggestions have also been made to reformulate the conception of determinism with respect to its application to physical law.

Causal completeness

See also: Causal closure

Physical determinism is related to the question of causal completeness of physics, which is synonymous with the weaker form of causal closure. This is the idea that every real event has a scientific explanation, that science need not search for explanations beyond itself. If causal completeness does not apply to everything in the universe, then the door is open to events that are not subject to physical law. For example, a relatively common view of mental events is that they are an epiphenomenon produced as a by-product of neurological activity, and without causal impact. In this case, only a failure of deterministic physical law would allow room for their causal significance.

Other formulations

A more modern formulation of physical determinism skirts the issue of causal completeness. It is based upon connections between 'events' supplied by a theory:

"a theory is deterministic if, and only if, given its state variables for some initial period, the theory logically determines a unique set of values for those variables for any other period."

— Ernest Nagel, Alternative descriptions of physical state p. 292

This quote replaces the idea of 'cause-and-effect' with that of 'logical implication' according to one or another theory that connects events. In addition, an 'event' is related by the theory itself to formalized states described using the parameters defined by that theory. Thus, the details of interpretation are placed where they belong, fitted to the context in which the chosen theory applies. Using the definition of physical determinism above, the limitations of a theory to some particular domain of experience also limits the associated definition of 'physical determinism' to that same domain.

See also

• Causal closure
• Determinism
• Epistemological pluralism
• Free will
• Mind–body problem
• Model-dependent realism
• Subject–object problem