Why the color of an emerald might be a riddle for all of science

Ideas & Philosophy · Published 3 October 2026 Swipe more like this →

In short: Nelson Goodman famously challenged our understanding of logic by asking how we know emeralds are green. His paradox of the predicate grue reveals that our reliance on past patterns is more fragile than it seems, forcing us to confront the deep, often hidden assumptions behind how we classify the world.

Nelson Goodman, an influential American philosopher born in 1906, left a lasting mark on logic, aesthetics, and the philosophy of science. While he is often remembered for his later work on art as a system of symbols, his most provocative contribution to analytic philosophy remains the 'new riddle of induction.' In his 1955 book Fact, Fiction, and Forecast, Goodman scrutinized the way we use past observations to predict future events, a process known as inductive reasoning.

Goodman’s challenge centers on the predicate 'grue.' Imagine a property that applies to all objects examined before a specific time, provided they are green, but also applies to objects not examined before that time if they are blue. Before that deadline, every emerald we observe confirms two competing hypotheses: that all emeralds are green, and that all emeralds are grue. Since both are equally supported by the evidence, Goodman argued that our preference for the 'green' hypothesis is not based on pure logic, but on our habits of language and classification.

This insight suggests that the problem of induction is far more complex than earlier thinkers like David Hume suspected. It implies that 'anything can confirm anything' unless we have a way to distinguish between law-like statements and arbitrary ones. This skepticism extended into Goodman's other work, including his development of nominalism—a project to remove set theory from the foundations of mathematics—and his founding of Harvard Project Zero, which explored how we understand and create art.

Ultimately, Goodman’s work pushes us toward a form of quasi-empiricism, where the focus shifts from individual, isolated truths to the theoretical networks we inhabit. By treating art as a symbolic language and questioning the very labels we attach to reality, Goodman forced philosophers to reconsider the scientific method itself, emphasizing that our understanding of the world is a constructive process rather than a passive observation.

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