Descartes has a Clarity First epistemology: (i) Clarity is a primitive (indefinable) phenomenal quality: the appearance of truth. (ii) Clarity is prior to other qualities: obscurity, confusion, distinctness – are defined in terms of clarity; epistemic goods – reason to assent, rational inclination to assent, reliability, and knowledge – are explained by clarity. (This is the first of two companion entries; the sequel is called, "Descartes's Method for Achieving Knowledge.").

This entry provides an overview of theories of concepts that is organized around five philosophical issues: (1) the ontology of concepts, (2) the structure of concepts, (3) empiricism and nativism about concepts, (4) concepts and natural language, and (5) concepts and conceptual analysis.