This page solves the “talking past each other” problem. “Determinism,” “causality,” and “predictability” are technical terms with precise meanings—but most debates never define them. Here we install a clean vocabulary (system boundary, isolated vs open, and reference frame) so the concepts can fit together logically before we even attempt to analyze “free will.”

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Fable Essay

Essay 1 of 5 in the Causality, Determinism, And Free Will reading path.

Arguments about free will have a signature failure mode — the introduction to this series preserves a famous specimen, in which two of the greatest public intellectuals of this century managed to disagree for over an hour without ever locating their disagreement. It wasn’t a failure of intelligence, and it wasn’t stubbornness. It was vocabulary. And because it happens to everyone — it would happen to you and me, right here, if we didn’t take precautions — this first essay is about installing the precautions.

Here, compressed, is the argument nobody was actually having:

“The universe is deterministic, so choice is an illusion.” “But nobody can predict what I’ll do next, so clearly it isn’t deterministic.” “We could predict what you’ll do next if we modeled your brain carefully enough.” “But then I could change my mind!” “Only because you knew you were being predicted!”

The suitcase word

“Determined” is a suitcase word: people pack it with whatever they happen to be carrying. Depending on the speaker, it means caused, or predictable, or inevitable, or morally excused, or no real choice. Two people can agree completely about the physics and still argue all night, because they unpacked different suitcases. Until we stop that semantic drift, there is no stable target to hit — and no hope of synthesis.

So let me unpack the suitcase slowly. Three definitions, installed once, used for the rest of this series:

• Determinism is a mathematical concept that describes a single-valued function (i.e. what makes a function a function). Feed a system’s state equation a time t and you get exactly one answer. One output for each input.
• Causality is a physical concept: the propagation of information forward in time. Later states carry the informational fingerprints of earlier states.
• Information conservation is the bridge idea most people are actually gesturing at with either word: in an isolated system, information is neither created nor destroyed. (It gets its own essay next.)

These terms are cousins, but not synonyms. Each will eventually get its own rigorous definition page; the job here is just to keep the conceptual map clean.

The ingredient everyone skips

Strictly speaking, determinism requires two ingredients:

1. Isolation — nothing outside the system reaches in (or vice versa);
2. Single-valued evolution — what’s inside evolves lawfully, one next state per current state.

Almost every debate leaps straight to ingredient two (usually via quantum mechanics) while skipping ingredient one. But isolation is logically prior: if the system isn’t sealed, outside influence defeats determinism no matter how well-behaved the internal laws are. The second question never even gets its turn. (Whether truly single-valued laws exist at all — whether the universe plays dice — is a deep swamp we’ll wade into elsewhere, wearing appropriate boots; or perhaps hip waders.)