An Idea (n.) is a perception of the mind which involves thinking of something instead of actuality (first hand) experiencing it.
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An Idea (n.) is made out of antecedent senses. It is not the result of causation but of conjunction.
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An Idea (n.) is the cognitive state that follows aphysn impression that provides you with a copy of the impression.
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An Idea (n.) for Hume is a chair in consciousness: the idea of a chair and the chair-as-idea are one and the same thing.
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An Idea (n.) is always ectypal, never archetypal — copied from objects; theism reverses this and gives thought the precedence.