Summary: An Essay Concerning Human Understanding (page 3)

Primary qualities are those that are utterly inseparable from the Body, in what estate soever it be; such as in all the alterations and changes it suffers, all the force can be used upon it, it constantly keeps ¹. They are found in every particle of Matter ².

These include extension, solidity, shape, motion, and number. Locke illustrates this with the example of a grain of wheat: if we cut it in half, each part still retains its extension, its shape, and so on.

These qualities give rise to simple ideas in us when we perceive them.

Secondary qualities are those that in truth are nothing in the Objects themselves, but Powers to produce various Sensations in us by their primary Qualities [Bulk, Figure, Texture, etc.] ³. These include colour, sound, taste, and so on.

Such qualities affect our senses through the action of imperceptible particles.

Primary qualities truly exist in bodies and so the ideas we have of them resemble reality; secondary qualities, by contrast, do not truly exist in objects, and the ideas we have of them do not correspond to anything real.

To illustrate this, Locke provides an example:

What is Sweet, Blue, or Warm in Idea, is but the certain Bulk, Figure and Motion of the insensible Parts in the Bodies themselves, which we call so ⁴.

It is commonly assumed that secondary qualities are truly present in things, and that what we perceive of them corresponds to reality. We believe, for instance, that blood is truly red. Or consider heat: it might seem absurd to claim that heat is not in fire. Yet if we bring a finger close to a flame, we feel pain—and no one would claim that pain is an actual property of fire. By the same token, heat is not an intrinsic quality of fire. In reality, heat is simply the motion of the particles that compose it; only this motion—a primary quality—is truly real.

An early version of the doctrine of primary and secondary qualities can be found in Descartes' Meditations on First Philosophy.

Where Descartes used the example of wax, Locke employs that of an almond pounded in a mortar: its colour and taste change, yet the only change the pestle brings about is in the almond's shape and extension. Once again, only the primary qualities of the object turn out to be real.

Locke also considers the possibility of third qualities: the capacity to produce an effect, such as the sun's ability to bleach wax or a match's ability to ignite fire. Such capacities are generally regarded as powers rather than qualities of the object; in reality, however, they are simply secondary qualities.

Locke goes on to examine various mental operations, including perception, memory, and abstraction. Only humans possess the capacity for abstraction; animals reason solely in terms of particular ideas.

Locke then introduces complex ideas, which we form by combining simple ideas. He gives several examples: Beauty, Gratitude, a Man, an Army, the Universe ⁵.

These fall into three categories: modes, substances, and relations.

Modes, unlike substances, do not exist independently but are Dependences on, or Affections of Substances ⁶—for instance, the idea of a triangle or that of murder.

A distinction is drawn between:

- Simple modes, which combine simple ideas of the same kind, such as the number twenty (1+1+1, etc.).
Mixed modes, which combine ideas of different kinds, such as beauty, which brings together ideas of form and colour.

- Modes have no independent existence, being simply the result of the mind combining simple ideas.

The complex idea of substance refers to something that exists independently—for example, the idea of lead, composed of the simple ideas of weight, colour, and so on.

Relation is the last sort of complex Ideas [...] which consists in the consideration and expressing one Idea with another ⁷.

Locke examines certain particular ideas—such as space, time, and number—to determine precisely how they are formed in the mind.

¹ II, 8, §9, p.134
² ibid
³ II, 8, §10, p.135
⁴ II, 8, §15, p.137
⁵ II, 12, §1, p.164
⁶ II, 12, §4, p.165
⁷ ibid., §7, p.166