Full Idea
We are told that the mind being able to consider each quality of things singly, or abstracted from those other qualities with which it is united, does by that means frame to itself abstract ideas.
Gist of Idea
The mind creates abstract ideas by considering qualities separated from their objects
Source
George Berkeley (The Principles of Human Knowledge [1710], Intro §7)
Book Reference
Berkeley,George: 'The Principles of Human Knowledge etc.', ed/tr. Warnock,G.J. [Fontana 1962], p.47
A Reaction
A helpful explanation of 'abstract' ideas. Berkeley gives colour and movement as examples. Fodor suggests that abstraction is the key strategy in empiricist epistemology. The difficulty is to decide whether the qualities are natural or conventional.