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Secondary qualities are thus intermediate cases, with some basis in the real world: “Colour and also temperature, hot and cold flavor, certain kinds of characterizations of sound, high-pitched and low-pitched in sound, there are a number of these,” Matthen said.

Although Descartes followed the ancient atomists in their distinction between primary and secondary qualities, he rejected their belief in empty space, the void.

The terminology we now use to express this distinction, between primary and secondary qualities, was introduced by Boyle in 1666 and popularized by Locke in 1689.

Primary qualities are objective; secondary qualities are subjective, in that they depend on our ways of sensing.

If the world was to be reducible to physical causation, then all mental experiences—intention, agency, purpose, meaning—must be secondary qualities, inexplicable within the framework of materialism.