Introduction to Philosophy
You will find this theory set out by John Locke in the 17th Century (Intro to John Locke)
I ask you to read just a bit of the first page of the extract reprinted in the Reader, p.209 paragraphs 7 and 8.
And I will read them for you a first time myself:
"7. To discover the nature of our ideas the better, and to discourse of them intelligibly, it will be convenient to distinguish them as they are ideas or perceptions in our minds; and as they are modifications of matter in the bodies that cause such perceptions in us: that so we may not think (as perhaps usually is done) that they are exactly the images and resemblances of something inherent in the subject; most of those of sensation being in the mind no more the likeness of something existing without us, than the names that stand for them are the likeness of our ideas, which yet upon hearing they are apt to excite in us.
8. Whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call idea; and the power to produce any idea in our mind, I call quality of the subject wherein that power is. Thus a snowball having the power to produce in us the ideas of white, cold, and round,- the power to produce those ideas in us, as they are in the snowball, I call qualities; and as they are sensations or perceptions in our understandings, I call them ideas; which ideas, if I speak of sometimes as in the things themselves, I would be understood to mean those qualities in the objects which produce them in us."
(Locke, An Essay Concerning Human Understanding, Bk II, Chapter VIII paragraphs 7 and 8, with subtitles removed; Reader, page 209; e-text)
You need to distinguish two things, says Locke. The ideas we form of objects in our minds and what causes them.
In his day, the notion of an idea was just transmogrifying into the concept of idea we have today. He is drawing on the old medieval conception of an idea when he says, as he does if you read paragraph 7 carefully, that you can think of an idea as (1) something that is in the mind and (2) a feature of the physical object which gave rise to perception in us. Nowadays we think of an idea as exclusively something mental, and emphatically not a physical feature of an object we may be perceiving. This concept of an idea as something exclusively mental is actually a conception we owe largely to Locke, but in this passage he is not articulating the Modern conception, but invoking the old one.
"To discover the nature of our ideas the better, and to discourse of them intelligibly, it will be convenient to distinguish them as they are ideas or perceptions in our minds; and as they are modifications of matter in the bodies that cause perception in us " (Reference above)
Footnote: Elsewhere, Locke explains that he intends to mean by an idea as " whatsoever is the object of the understanding when a man thinks ". We take him today to be meaning by this something exclusively mental, and we look back on Locke as one of the great sponsors of this Modern conception - the conception of an idea as something exclusively mental.
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