Helen asked:

Why is the difference between primary and secondary qualities that Locke so emphasized rejected by
David Hume and latter British empiricists?

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The short answer to this question is that Locke's distinction is perfectly sound, but Berkeley and
Hume had their own agenda. In pushing that agenda, the primary-secondary quality distinction was
the obvious target to attack.

Some qualities of a thing, such as mass, volume, shape are possessed by those things irrespective of
the existence of subjects capable of perceiving those qualities. Other qualities, such as colour, smell,
taste only exist by virtue of the power of the thing that possesses that quality to bring about
experiences in a percipient subject. That, in essence, is the distinction between primary and
secondary qualities.

For both Berkeley and Hume, however, the very idea of primary qualities was inconsistent with the
metaphysical principle that what we term 'material objects' are, in reality, constructed out of the
materials of experience. For Berkeley, statements about physical objects reduce to hypothetical
statements about the experiences that one might receive in such-and-such circumstances. In Hume's
'ideal' theory, the notion of a material object existing distinct from perception, and continuing to exist
during periods when it is not perceived is self-contradictory. 'Material object' is a useful fiction, nothing
more.

It therefore follows from Berkeley's and Hume's theories that from a metaphysical point of view all
qualities are, ultimately, secondary qualities.

Geoffrey Klempner