According to Locke, primary qualities are qualities like extension, figure, motion or rest, and number. These “inhere” in bodies; that is, the simple ideas produced in human minds by them somehow resemble them. The idea here is that the collection of corpuscles that we perceive as a cup really is shaped like a cup; we perceive certain heaviness when holding it, and it really does weigh so many ounces; we see it moving at such and such speed, and it really is moving. This is the sense in which our ideas resemble the object. Secondary qualities are those constitutions of a thing’s “minute and insensible parts” which produce certain sensations in us, such as colors, tastes, and the rest; yet with them, there is no resemblance between the cause of our sensations and the sensations or ideas themselves. For example, we perceive blueness, but the object we are looking at is not really blue; what it really has is a certain surface structure with the power to absorb all but the blue range of light. Thus, we perceive extension and the body is really extended, while with respect to a fire, for example, we perceive yellowness or warmth, but the fire is not really yellow or warm (though it may have “high” temperature or temperature which is usually correlated with the experience of warmth); rather, the fire, thanks to its internal constitution (plasma, as we now know), has a power to appear to an observer as yellow and warm. (An Essay Concerning Human Understanding, II, viii, 15)
Just as it is obvious that pain one feels from being too close to the fire is not actually in the fire, neither is warmth, which is merely a sensation generated by a certain power of the fire. A warm hand put in lukewarm water feels cold, while a cool hand put into the same water, warm. This implies that if heat as it is experienced were really in the water, the water would have to be both cold and warm at the same time, which is impossible. Heat then is relative to the perceiving mind; the same thing can seem warm to one person and cold to another. So, the secondary quality of temperature has to do with the quickness of movement of the molecules in a substance, and it is capable of producing different sensations in different observers.
The operation of the physical causes of secondary qualities is mysterious, at least to the science of Locke’s day; these operations “are hid from us, in some things by being too remote, and in others by being too minute.” (IV, iii, 24) Locke thinks that we can never know the underlying arrangement of corpuscles that cause our ideas of secondary qualities. This is because all of our ideas come from either experience and reflection, and the insensible parts are simply too tiny to see. (Reflection does not apply here at all.) We cannot predict what effects the minute parts of an object will have on us; nor can we, having seen the effect, know how it was produced. (IV, iii, 25) Finally, we cannot know how the structure of our immaterial minds permits us to experience ideas caused in us by material corpuscles. (IV, iii, 28)
It follows that only the ideas of primary qualities really exist. (II, viii, 17) They are “real” in the sense that they persist in the object even if there is no one around to observe it. But the ideas of secondary qualities vanish as soon as the observer, so to speak, leaves the room. These qualities are reduced back to their causes: an array of corpuscles making up the object with the power somehow to produce ideas in us yet which do not in any way resemble these ideas.