Physical determinism

Physical determinism is a position in philosophy that holds that all physical events occur as described by physical laws. A narrower definition states that physical determinism 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. The emphasis upon a particular time in the last definition may be too specific to encompass all forms of physical determinism and, as is discussed below, a broader view requires only that events be logically connected by a physical theory.

These definitions leave open just what "physical" might include. So, for example, one could limit the scope of 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

These limitations of physical determinism can be expressed in terms of its origin in a third-person perspective, from which "thoughts, feelings, and desires are not physical events":

"Epistemically, the mind is determined by mental states, which are accessible in First-Person Perspective. In contrast, the brain, as characterized by neuronal states, can be accessed in Third-Person Perspective. The Third-Person Perspective focuses on other persons and thus on the neuronal states of others' brain while excluding the own brain. In contrast, the First-Person Perspective could potentially provide epistemic access to own brain...However, the First-Person Perspective provides access only to the own mental states but not to the own brain and its neuronal states."   —Georg Northoff Philosophy of the Brain: The Brain Problem, p. 5

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 governed by the past or present according to all-encompassing deterministic laws. However, in this article this practice is avoided.

Causal completeness

The scope of physical determinism is related to the question of the causal completeness of science, 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 determinism. For example, 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" still to be invented.

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 mental or psychological world, the world of our feelings of pain and of pleasure, of our thoughts, of our decisions, of our perceptions and our observations; in other words, the world of mental or psychological states or processes, or of subjective experiences.
• World 3: the world of products of the human mind, including art, science, and religion.

World 3 includes physical theory as a particular case. But World 3 is a creation of the human imagination, and such acts of imagination are a part of World 2. Accordingly, one could argue that physical determinism is a child of the imagination, and although physical determinism has its successes in describing World 1, it may not apply to World 2 or World 3. The subjective aspects of theories contained in World 3 are not readily framed within the third-person perspective of science used to explain World 1. These subjective aspects are described in the article Subjective-objective dichotomy.