Rice Is Rivalrous; Bruckner Is Not

Paul Samuelson is usually credited with bringing the theory of public goods into modern economics, but the Stanford Encyclopedia of Philosophy notes the idea predates him, reaching back to John Stuart Mill, the Italian public-finance writer Ugo Mazzola, and the Swedish economist Knut Wicksell. Samuelson's own term was a mouthful: he defined a collective consumption good as one that all enjoy in common, in the sense that each individual's consumption of it leads to no subtraction from anyone else's consumption of the same good. Contemporary economics has a tidier label for that feature: non-rivalry. The definition runs on subtraction rather than popularity. A good is rivalrous exactly when one person's consumption diminishes someone else's ability to consume it. The encyclopedia's two examples do the work better than a definition can. Bob's consumption of a grain of rice makes it impossible for Sally to consume that same grain — the object is used up, so rice is rivalrous. Sally's enjoyment of Bruckner's Symphony No. 9, by contrast, does not diminish Bob's ability to enjoy it. Rice is consumed once; one person enjoying the symphony does not use up another person's opportunity to enjoy it.