Locke on the
distinction between primary and secondary qualities
We have already had some occasion to concern ourselves with this
distinction which Locke makes in Chapter VIII between primary and secondary qualities.
Descartes makes a similar distinction in the wax example in the Meditation II section 11. The distinction itself goes back to
the Greek atomists. In spite of its antiquity, however, the distinction
is so important to modern philosophy (Descartes to Hume) that David Hume
says it characterizes modern philosophy.
The distinction between primary and secondary qualities captures the
difference between the ways things are independently of how we perceive
them, and those qualities of things which depend on how we perceive them.
Thus, a tower or mountain presumably occupies space and remains
(relatively) at rest, whether we perceive it or not. But the fact that
the mountain appears purple from a distance is an artifact of the
interaction of the object with a particular kind of perceptual apparatus.
Dogs, for example, have no color receptors in their eyes, thus they see
the world in shades of black and white. Human beings, who do have color
receptors, see the world as a multiplicity of colors. Without a human
perceptual apparatus, there would be light rays, and absorption of light
rays, but no perceived color.
What are the primary qualities, and what are the secondary ones?
Extension, figure, number and motion of bodies large enough to be observed
is the list which Locke gives first. (II. 8, 12) These qualities produce in us the ideas
of primary qualities. The secondary qualities are colors, such as white,
violet and green; tastes, smells, and sounds. The primary qualities
produce the ideas of secondary qualities by interacting with our
perceptual apparatus. Thus without eyes and brains/minds, while there
would be objects which absorb various light rays and light, there would be
no perceived color. Without noses, nerves, brains and minds there would be
no perceived smells and so on.
Locke goes on to observe (II, 8, 15) that the ideas of primary qualities
resemble the properties in the objects which they represent, while this is
not true of secondary qualities. Thus he says: "...that the ideas of
primary qualities of bodies are resemblances of them, and their patterns
do really exist in the bodies themselves, but the ideas produced in us by
these secondary qualities have no resemblance of them at all."
The distinction connects skepticism, science and philosophy in
interesting ways. One of the strands of skeptical thinking holds that the
variability of our perceptions shows that they are mind dependent, and
because reality is determinate, this variability shows that we are
grasping appearance and not reality. Thus if you see an oar as bent and I
see it as straight, we know that it cannot be both, it must be one or the
other. But since we see it both ways, we have appearance and not reality.
In fact, one of Locke's most famous examples (used by Berkeley as well)
-- that of dipping a hand which is warm and the other hand which is cold
into a bucket of water -- we find in Montaigne. But the scientist differs
from the skeptic in holding that we can distinguish appearance from
reality, we can tell which properties of things they have independently of
us, and which depend on our perception. So, the skeptic sees all
properties as appearance, while the scientists see only some properties as
apparent, and others as representing reality. Thus, the distinction
between primary and secondary qualities implies that we can know objective
reality in a way which the skeptic denies is possible.