Primary and secondary qualities

The distinction between the primary and secondary qualities of objects was an important theme within British empiricist philosophy, the three key proponents being John Locke, George Berkeley and David Hume.

Locke sets out the distinction in his 'Essay Concerning Human Understanding'. The distinction applies to the "simple" ideas we receive from sensation.

Primary qualities are properties objects have that are independent of any observer and include such properties as solidity, extension, motion, number and figure. They exist in the thing itself, can be determined with certainty, and do not rely on subjectivity. Secondary qualities are properties that produce sensations in the observer, such as colour, taste, smell, and sound. They are effects things have on people. Knowledge that comes from secondary qualities does not provide objective facts about things.

Locke argues that secondary qualities are somehow caused by the arrangements of matter, matter has the power to cause the ideas of secondary qualities. He believes that there is scientific support for his theory based on Boyle's new 'corpuscular hypothesis'.

Locke supports his theory by the use of examples. I will quote directly from Chapter VIII of Book II:
"Take a grain of wheat, divide it into two parts; each part has still solidity, extension, figure, and mobility: divide it again, and it retains still the same qualities; and so divide it on, till the parts become insensible; they must retain still each of them all those qualities. For division (which is all that a mill, or pestle, or any other body, does upon another, in reducing it to insensible parts) can never take away either solidity, extension, figure, or mobility from any body, but only makes two or more distinct separate masses of matter, of that which was but one before; all which distinct masses, reckoned as so many distinct bodies, after division, make a certain number. 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.

...Secondary qualities of bodies. Secondly, such qualities which in truth are nothing in the objects themselves but power 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, &c. These I call secondary qualities. To these might be added a third sort, which are allowed to be barely powers; though they are as much real qualities in the subject as those which I, to comply with the common way of speaking, call qualities, but for distinction, secondary qualities. For the power in fire to produce a new colour, or consistency, in wax or clay,- by its primary qualities, is as much a quality in fire, as the power it has to produce in me a new idea or sensation of warmth or burning, which I felt not before,- by the same primary qualities, viz. the bulk, texture, and motion of its insensible parts."

Locke's theory of perception is a 'mediated theory of perception'. Accordingly, we do not have direct access to the world around us, but rather this access is mediated through our ideas.