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What we see depends on the light entering the eye. Furthermore we do not even perceive what enters the eye. The things transmitted are waves or—as Newton thought—minute particles, and things seen are colours. Locke met this difficulty by a theory of primary and secondary qualities. Namely, there are the primary qualities, and there are other things which we perceive, such as colours, which are not attributes of matter.
Why should we perceive secondary qualities? It seems an extremely unfortunate arrangement that we should perceive a lot of things that are not there. Yet this is what a theory of secondary qualities in fact comes to. There is now reigning in philosophy and in science an apathetic acquiescence in the conclusion that no coherent account can be given of nature as it is disclosed to us in sense-awareness without dragging in its relations to mind. The modern account of what the mind knows of nature is not, as it should be, merely an account of what the mind knows of nature; but it is also confused with an account of what nature does to the mind.
[…] The ground taken is this: sense-awareness is an awareness of something. What then is the general character of that something of which we are aware? We do not ask about the percipient or about the process, but about the perceived.—Alfred North Whitehead
The Concept of Nature, ch 2
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