Human concept use routinely violates necessary-and-sufficient-condition logic.

Evidence

There is a huge cognitive psychology literature on concepts and adjectives. It is clear that people’s—including social scientists’—manipulation of concepts and adjectives often does not conform to the classic necessary and sufficient condition logic; for a good review see Murphy 2002. — Gary Goertz, Social Science Concepts and Measurement, 1507

Categorization

• XREF: Connects to Eleanor Rosch’s prototype theory and to Wittgenstein’s family-resemblance account of concepts, which this book’s later chapters will build on.
• DEFINE: Flags the classic necessary-and-sufficient-condition definition — a concept whose members all share the same defining attributes — as the target being rejected.