The Reality of Abstractions

Definition

Deutsch's criterion for reality is explanatory: something is real if it is referred to, ineliminably, by our best explanation of something — regardless of whether it is physical. High-level phenomena (boiling water, a chess-winning computer program, a prime number) often admit good explanations that do not reduce to, and cannot be replaced by, lower-level physical descriptions, even though they are physically instantiated by lower-level processes. Deutsch calls this emergence, and uses it to reject reductionism (the doctrine that only the lowest-level physical description is truly explanatory) as itself a bad, dogmatic philosophy rather than a discovery of science.

In the Book

Deutsch's key illustration is a copper atom in the tip of the nose of a statue of Winston Churchill: the only real explanation for why that specific atom sits there involves Churchill's wartime leadership, the tradition of erecting statues, and the properties of bronze — high-level historical and cultural facts — not a physical trajectory-by-trajectory account of every atom involved, which "would explain nothing" even if it were computable. He extends this with Douglas Hofstadter's thought experiment of a computer built from spring-loaded dominoes that computes primality: asked why one particular domino never falls, the true explanation is "because 641 is a prime number" — a fact about abstract mathematics, not physics — even though the domino network is a physical system obeying only physical laws. Deutsch also cites general relativity's denial of a literal "force of gravity" (replaced by curved spacetime) and the second law of thermodynamics (which has never been derived from a simple low-level statement about individual atoms) as cases where good explanation runs orthogonal to, or even against, naive reductionism. He extends the same reasoning to morality and aesthetics: since "right and wrong" appear ineliminably in our best explanations of what to do, they too must be treated as real.

Why It Matters

This licenses taking seriously — as real, not just useful fictions — the higher-level entities that actually do the explanatory work in any field: institutions, incentives, culture, meaning, "the market," a piece of software's behavior, a person's intentions. It blocks the common move of dismissing an explanation as "not really real" just because it isn't stated in terms of the most fundamental substrate, while still holding it to the same standard: it counts as real only if it's actually indispensable to a good explanation, not merely convenient.