Physical determinism

Physical determinism is a position in philosophy that 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 different words, physical determinism holds that all physical events occur as described by physical laws.

Sometimes the restrictions to physical events and physical laws are ignored, and physical determinism is used as a synonym for nomological determinism, that all future events are determined by the past or present. However, in this article this practice is avoided.

The initial definition leaves open just what "physical" might mean, or a "complete" description. So, for example, one could take physical determinism as

"a concept only relevant to the mathematical models of physics and other physical sciences, although its relevance to the world of everyday choice and action is questionable...if thoughts, feelings, and desires are not physical events, it is unlikely that physical theories are appropriate models for thinking about such nonphysical events." —Robert C. Bishop Chaos, indeterminism, and free will, p. 84

Causal completeness

The scope of physical determinism is related to the question of the causal completeness of science, the idea that every event falls within the domain of scientific explanation. If causal completeness does not apply to the universe, then the door is open to events that are not subject to physical determinism. A common view of mental events is that they are an epiphenomenon only correlated with neurological activity, and without causal impact. However, a failure of physical determinism would allow room for their causal significance.

A more general 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. That limitation leaves open the question whether there is a physical "theory of everything".

Popper's three worlds

A much more general reservation about the applicability of physical determinism stems from the formulation of Popper's three worlds. Popper split the world into three categories:

• World 1: the world of physical objects and events, including biological entities
• World 2: the world of mental objects and events, including conscious and subconscious experiences
• World 3: the world of products of the human mind, including art, science, and religion.

World 3 is a creation of the human imagination (a part of World 2), and includes physical theory as a particular case. Accordingly, one could argue that physical determinism is a child of this process, and although it has its successes in World 1, it may not apply to World 2 or World 3. The subjective aspects of World 3, which are not readily subsumed in the third-person perspective of science used to explain World 1, are described in the article Subjective-objective dichotomy.