As part of his ‘Essay Concerning Human Understanding’, Locke considers the question of primary and secondary qualities. The next two posts present a section-by-section breakdown of the relevant discussion.
Section 8: Locke defines a couple of key terms relating to his arguments. An ‘idea’ is an immediate object of thought, perception or understanding that occurs in the mind. If an object has the ability to produce related ideas in the mind, these abilities are called ‘qualities’. In other words, object A has quality B that gives rise to idea C. For Locke, the mental side of a quality is important, so similar to Boyle, he supports a dualist interpretation of qualities.
Section 9: The most obvious type of qualities are those that are ‘utterly inseparable’ from the body, no matter how much force is applied. Locked uses the example of a grain of wheat that is divided into two. Each part still possesses solidity, extension, figure and mobility. Division can never remove these qualities entirely. Locke calls these original or primary qualities of bodies.
This interpretation is slightly more extensive than that of Boyle’s. Boyle only recognised size and shape as primary qualities, where Locke is prepared to also recognise the idea of ‘number’. By ‘number’, Locke is referring to the number of elementary particles that exist in the object, as opposed to the more familiar count of objects. Locke is also willing to accept that primary qualities exist at the microscopic level, which Boyle didn’t specify.
Section 10: Locke moves on to secondary qualities. These are powers in the objects to produce a certain sensation within the observer. The power itself is the secondary quality. Locke also introduces a third type of quality, an ability to alter the composition of another object. He will return to this third type in Section 23.
Sections 11/12/13: In these sections Locke considers how the qualities in objects have the ability to create ideas. This is due to ‘motion’, both in the bodies themselves and also within the observer. Both primary and secondary qualities are derived from this motion. So when we consider something like a violet, the smell and colour do not resemble the quality at all. Similar to Boyle, Locke uses the example of pain to show that even though the ‘pain’ does not resemble anything in the object itself, God has set up a system whereby certain motions of particles result in certain kinds of ideas being produced in the mind.
Section 14: Locke makes a similar point to Boyle in that we might commonly think that secondary qualities exist solely in the objects themselves, whereas all the object contains is the power to produce the relevant motion within us. However, Locke does recognise that these powers arise from the primary qualities (which in turn, are based on the motion of the elementary particles).
Both Locke and Boyle speak of a quality existing ‘IN’ an object. By using this preposition, Locke seems to be suggesting a purely visual sense of seeing the quality in question. Because we can see size and shape, these are considered primary qualities, but because we can’t see any secondary qualities, these do not exist in the same way as primary qualities.
Section 15: Locke makes a distinction between primary and secondary qualities in terms of resemblance. He asks that we compare the idea in our mind to that of the quality. If the idea resembles the quality, then we can be confident that this is a primary quality. If however the idea has no resemblance to the quality, then it must be a secondary quality. For instance, a smell or a taste does not resemble anything in the object, so these must be secondary qualities.
Section 16: Locke uses the example of fire to show how we perceive secondary qualities. If we sit too close to the fire, then we will get burnt, but if we sit at the right distance, then we will get warm with no discomfort. It is clear that the same quality of fire applies in both cases, so if we make a conclusion about one case, we ought to make the same conclusion about the other. If we conclude that pain does not resemble any quality in the fire, then we have to say the same thing about the warmth. Both are secondary qualities.