Re: Colour perception

From: Stevan Harnad (
Date: Mon Feb 24 1997 - 21:36:46 GMT

> Date: Mon, 24 Feb 1997 09:32:29 GMT
> From: elizabeth lee <>
> Subject: Colour perception
> To:
> I am working on an assignment for the perception course and wish to
> look at the above, particularly cerebral achromatopsia. I am basing the
> essay on a case in one of Oliver Sacks' books. The problem is, I'm
> having difficulty understanding the concept of colour perception, as
> with all these things, I feel I've almost got it but it doesn't ever
> seem to slot into place. I've got the basic texts, (Perception, Visual
> Perception etc.) but still don't understand the difference between
> perceiving colour (is there any such thing?) and perceiving
> wavelengths. Can you point me in the direction of a text which might
> help?

Photons are electromegnetic waves. When they bounce off certain surfaces
and hit your eye, you see colour. Seeing different colours is caused
by having light of different wave-lengths hit your eye. What you SEE
is red; what causes it is light of a certain wave length.

The philosopher John Locke distinguished between what he called
"primary" and "secondary" qualities. The primary ones -- like distance,
weight, shape -- he said were real and physical. The secondary ones --
he used colour for his example -- are not really physical qualities.
They are CAUSED by physical qualities (wave length is a physical
quality) but the effects are mental. Red looks red only in your mind.

This is just another version of the mind/body problem: How can matter
(with size or weight or wave-length) be related to mind (with colour or
what it FEELS like to lift a stone of a certain weight or what a shape
LOOKS like -- as opposed to what primary [geometric] qualities it

That will only help a bit. You should now know the difference between
colour and wave-length; it's just like the difference between what
a stone weighs and what it feels like to lift it.

But because reconciling primary and secondary properties (matter and
mind) has not yet quite been successfully accomplished despite our
distance from Locke's 17th century, some of your puzzlement is

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