 Instead of trying to establish a canonical doctrine of primary and secondary qualities, it is better to recognize the close temporal and conceptual connection between seventeenth century variants of the distinction and the innovative non-Aristotelian theories of matter and sense perception inspired by Kepler’s amendments to the Copernican astronomy. This article considers the seventeenth and eighteenth century versions which have most influenced recent analytic philosophy. There are two reasons for this restriction. First, the historical debates are nuanced and subject to several interpretations; comparison helps to focus the questions they are meant to answer. Second, more recent work on particular secondary qualities is heavily influenced by diverse philosophical movements and changing views of the relation between philosophy and the sciences which cannot be adequately considered here (see Ross 2015). This entry uses the following notational conventions: “PQ” stands for “primary quality”; “SQ”, for “secondary quality”; additional designations are formed in obvious ways. The name of one PQ or SQ is often used to indicate all qualities of the same type, e.g., “red, etc”. indicates red and the other SQs.)