Qualities thus considered in bodies are:
First, such as are utterly inseparable from the body… These I call original or primary qualities of body; which I think we may observe to produce simple ideas in us, viz. solidity, extension, figure, motion or rest, and number…
Secondly, such qualities which in truth are nothing in the objects themselves but powers to produce various sensations in us by their primary qualities, i.e. by the bulk, figure, texture, and motion of their insensible parts, as colours, sounds, tastes, etc.
Book 2, Chapter 8, 9–10
Tops spin or rest motionless in place. They are long or short in length. Many have a round shape. These are properties of tops and other physical things.
Locke thought that these kinds of properties, such as motion, size, shape, and number, are primary: they are inherent in the objects.
A colour wheel, on the other hand, has segments with different colours. Some segments might be red, and others green or blue. Locke thought these kinds of properties, such as colour, taste, smell, and sound, are secondary: they depend on a person’s subjective sensory experience.
For Locke, secondary qualities, while real properties of an object, are not inherent in the objects. Rather, secondary qualities are powers an object has to produce certain sensations in us. A sharp needle may prick us and cause us pain. We say the needle is painful, but we don’t think being painful is an inherent property of the needle. Needles merely have the power to cause us pain.
Locke thought colours were like being painful and unlike being round.
Image credit: Cliff Landesman
Listen to audio commentary by philosophers Tim Crane and Peter Millican, with Locke's words read by Victoria Jenner.
Download the accompanying transcript for the primary and secondary qualities audio commentary (Word document)