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Full Idea
Sensible qualities are by philosophers divided into primary and secondary; the former are extension, figure, solidity, gravity, motion and rest, which exist really in bodies.
Gist of Idea
Primary qualities (such as shape, solidity, mass) are held to really exist, unlike secondary qualities
Source
George Berkeley (Three Dialogues of Hylas and Philonous [1713], I p.169)
Book Ref
Berkeley,George: 'The Principles of Human Knowledge etc.', ed/tr. Warnock,G.J. [Fontana 1962], p.169
A Reaction
A crucial distinction, which anti-realists such as Berkeley end up denying. I think it is a good distinction, and philosophers should fight to preserve it.