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Sue asked:

What exactly was Berkeley's Doctrine of Abstract Idea? What was its implication in his overall
idealism?

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There was a similar question from Glouisel which you can find in Answers 11 (Answer to Glouisel).

Berkeley did not believe in "abstract ideas" and argued against them, especially in relation to Locke,
in great detail at the beginning of Principles of Human Knowledge. For Berkeley an idea is a sensory
item or a copy of a sensory experience in memory or imagination. Things or objects are complexes of
ideas since to have an idea of a table requires that you both see it and feel it, but this is not an
abstraction from sensation. Berkeley thought that Locke's idea of a triangle which is abstract and not
an idea of any sort of particular triangle was absurd. Rather, for Berkeley, an abstraction of something
like a triangle is a theoretical construction which belongs properly to language rather than perception.
We can speak of a triangle in general but cannot have an idea of a triangle in general and language
misleads us into thinking that we can imagine or conceive of some abstraction of a triangle. Language
does not determine what there is but what we perceive does.

When we perceive a tree, this is a complex of ideas. We see a tree but this visual idea doesn't give us
an idea of its solidity. For that we need an idea from the tactile sense, but the resistance we find on
touching tree doesn't provide us with an idea of matter as substratum. Berkeley rejects Locke's
distinction between primary and secondary qualities and argues that so-called primary qualities are
sensible properties. A sensible property is essentially perceived, so things exist because they are
perceived by God.

Rachel Browne