Determinism

What determinism says

Take any complete description of the universe at time t₀ — every particle's position, every field's value, every photon in flight. Add the laws of nature. Determinism claims these together logically entail the complete description of the universe at every later (and earlier) time. Schematically:

P(t₀) & L ⊢ P(t) for all times t.

The thesis has three components, each individually contestable:

1. There is a complete state. At every moment, there is a definite fact of the matter about the configuration of the universe. (Quantum mechanics on most readings denies this for unmeasured systems.)
2. There are laws. Some non-empty set of regularities governs how states evolve.
3. The laws are deterministic. They map any one state to a unique successor. (Stochastic laws — like the Born rule of QM — fail this.)

Determinism is a claim about nomological necessity: given the actual laws, only one future is physically possible. It is not a claim about metaphysical necessity — the laws themselves could have been different in a different possible world.

Laplace's demon

In A Philosophical Essay on Probabilities (1814), Pierre-Simon Laplace put it like this:

“An intelligence which, at a given instant, would know all the forces that animate nature and the respective situations of the beings which compose it... would embrace in the same formula the movements of the largest bodies in the universe and those of the lightest atom. For such an intelligence, nothing would be uncertain, and the future, like the past, would be present before its eyes.”

The demon is a heuristic, not a metaphysical commitment. Even if determinism is true, no actual computer could be the demon — to simulate the universe with full fidelity you'd need a computer at least as large as the universe. The demon's epistemic feat is consistent with determinism but not implied by it.

Kinds of determinism

The unqualified word covers several distinct positions.

• Causal determinism. Every event has sufficient causes, and the same causes always yield the same effects. The classical version, defended by d'Holbach (System of Nature, 1770).
• Logical / theological determinism. Future events are already true now (Aristotle's sea battle), or already known to God. The fixity is semantic, not physical.
• Biological / genetic determinism. Behaviour and traits are fixed by inheritance. A weaker, domain-specific claim, often empirically dubious.
• Environmental determinism. Outcomes are fixed by environment. Behaviourist tradition (Watson, Skinner).
• Psychological / motivational determinism. Choices are fixed by the agent's strongest desire. Hobbes, mechanistic moderns.
• Theological predestination. God determines the spiritual fate of each soul (Calvin, Augustine on grace). Distinct from causal determinism, though often conflated.

Determinism vs neighbouring theses

	Determinism	Fatalism	Predestination	Predictability	Indeterminism
One possible future?	Yes	Yes (relative to outcome)	Yes (for soul)	—	No
Causes matter?	Yes — deliberation included	No — outcome bypasses causes	God's will	—	Yes, but underdetermine
Free will compatible?	Compatibilists: yes	Almost no one defends	Disputed (Augustine)	Trivially	Libertarians: yes
Empirical or metaphysical?	Both — depends on physics	Metaphysical / theological	Theological	Empirical	Both
Champion	Laplace, Spinoza, d'Holbach	Stoics (some readings)	Calvin, Augustine	Engineers, weather	Epicurus (swerve), James
Modern physics fit	Newtonian, GR — yes; QM — debated	None	None	—	Copenhagen QM, GRW

Worked example: a billiard break

A cue ball strikes the rack with a measured velocity v at angle θ. Classical mechanics + Newton's laws + the table's friction coefficient + the masses of the balls jointly determine where every ball comes to rest. In principle, given enough precision, you can replay the break a million times and get identical outcomes. This is paradigmatic determinism.

Now perturb the initial velocity by 10⁻¹⁰ m/s. After a few collisions the trajectories diverge dramatically — Michael Berry calculated that to predict 9 collisions of a billiard break, you'd need to account for the gravitational pull of an electron at the edge of the galaxy. The system remains deterministic (each rerun from identical initial conditions gives the same outcome) but is not predictable in practice. This separation is the standard reply to “quantum chaos” arguments: chaos doesn't refute determinism, it just defeats prediction.

Determinism and modern physics

Newtonian mechanics, special relativity, general relativity, and classical electromagnetism are all deterministic in their standard formulations. Trouble arrives with quantum mechanics. The standard (Copenhagen) interpretation includes wavefunction collapse, governed by the probabilistic Born rule — given the same prepared state, repeated measurements yield genuinely different outcomes. Bell's theorem (1964) rules out local hidden-variable theories that could restore determinism by adding extra parameters.

But two major interpretations restore determinism at a price.

• Bohmian mechanics (de Broglie–Bohm pilot wave). Particles have definite positions guided by a deterministic wavefunction. Apparent randomness comes from ignorance of initial conditions. Cost: explicit non-locality.
• Everettian / many-worlds. The universal wavefunction evolves deterministically per the Schrödinger equation; apparent collapse is decoherence into branches. Each branch contains a different outcome; none is “the” outcome. Cost: vast ontological commitment to unobservable branches.

The empirical question isn't settled. The philosophical question — whether any indeterminism, even at the quantum scale, is enough to block the consequence argument against compatibilism — gets discussed independently.

Objections to determinism

• The phenomenological objection. When deliberating, we feel an open future — multiple options seem genuinely available. Determinism makes that feeling illusory.
  Reply. Compatibilists argue the felt openness is consistent with determinism: it tracks our ignorance of which option we'll select, not any branching in the world.
• The QM objection. Bell's theorem and standard QM look indeterministic; the empirical evidence weighs against determinism.
  Reply. Bohmian mechanics and Everettian QM are both deterministic and empirically equivalent to Copenhagen; the issue is interpretive, not settled by data.
• The agent-causal objection (Chisholm). Persons are sources of causation that aren't fully reducible to prior events. Determinism rules this out by fiat.
  Reply. Determinists ask for an account of what an agent-cause is that doesn't simply rename ordinary brain processes.
• The pragmatic objection. Belief in determinism is correlated with reduced moral effort, increased cheating (Vohs & Schooler 2008). It is dangerous to teach.
  Reply. Truth is not graded by social effect. If determinism is true, the practical question is how to teach it without reducing pro-social behaviour, not whether to deny it.

Why determinism matters

• Free will. The compatibility question — whether free will survives if determinism is true — is the central long-running debate.
• Moral and legal responsibility. Hard determinists (Pereboom, Caruso) argue retributive punishment can't be justified if determinism is true. Compatibilists disagree. The practical stakes for justice systems are real.
• Religion. Predestination, divine foreknowledge, and providence all interact with determinism. Calvinist, Thomist, and Molinist positions stake out the options.
• Science. Whether the fundamental laws are deterministic shapes how we model emergence, complexity, and the relation between micro and macro.
• AI and prediction. Even granting determinism, computational irreducibility (Wolfram) means the only way to know what a complex deterministic system does is to run it — there's no shortcut formula.

Common confusions

• Determinism ≠ fatalism. Determinism gives a causal role to your deliberation; fatalism bypasses it (“you'll die in Samarra no matter what road you take”). The deliberation that talks you out of Samarra is itself part of the deterministic chain.
• Determinism ≠ predictability. Chaotic deterministic systems (weather, billiards) are unpredictable in practice while remaining deterministic in principle.
• Determinism ≠ predestination. Predestination is a theological claim about salvation; determinism is a metaphysical claim about physical states.
• Determinism ≠ reductionism. A non-reductive view can still be deterministic (mental events fully determine each other without reducing to physics).
• “If determinism is true, why deliberate?” Because deliberation is the cause that determines your choice. The question presupposes the “why bother” fallacy — that effort is wasted if outcomes are fixed, when the effort is part of what fixes them.