Locke on Primary and Secondary Qualities

I want to take one more opportunity clarify this important distinction. In itself, the distinction is not hard to understand, but the language we use tends to blur the very distinction Locke is trying to explain, and all-too-often leads to misunderstandings of the point Locke is trying to make.

Let me start, as I did in class, with a quote from the text:

Whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call idea; and the power to produce any ideas in our mind, I call quality of the [object]* wherein that power is.

(*Locke says “subject” where I put “object.” I changed it as to avoid confusion, as he doesn’t mean anything like a “mind” or conscious subject here, but only “a thing which has properties,” i.e., an object.)

The important point to remember here is that ideas and qualities are different things. Ideas are “mental entities,” things that exist “in our minds,” whereas qualities are the causal properties (“powers”) of physical objects in virtue of which they can cause ideas to exist in our minds, i.e., in virtue of which they can cause us to have certain kinds of sensations. Ideas are sensation in our mind and qualities are the properties of objects that cause us to have those sensations.

Given that qualities are causal properties of objects, it would seem that any distinction between kinds of properties—such as the distinction between primary and secondary qualities—should be a difference in the kinds of properties a thing has. This is correct, but remember that Locke defines “qualities” as powers to produce ideas in our minds, and so that different kinds of qualities (in the object) are going to be distinguished in terms of the different kinds of ideas they produce in our minds. The difference between primary and secondary qualities, then (or, at least, the crucial one) is in the kinds of ideas they produce in our minds. What is that difference?

Locke explicitly answers that question:

From what I have said, I think it easy to draw this observation,--that the ideas of primary qualities are resemblances of them [i.e., that the ideas are “resemblances” of the qualities], and their patterns do really exist in the bodies themselves, but the ideas produced in us by these secondary qualities have no resemblances to them at all [i.e., the ideas do not “resemble” the qualities].

So the difference between primary and secondary qualities can be described in terms of differences in the ideas that they produce in our minds. Qualities, by definition, are the powers objects have to produce ideas in our minds. The primary qualities of objects produce ideas in our minds that “resemble” the corresponding qualities in the objects that caused us to have those ideas. The secondary qualities of objects produce ideas in our minds that do not resemble the corresponding qualities in the objects that produced those ideas in our minds.

An example: I perceive a blue rectangle (i.e., a blue rectangular object). According to Locke, shape (being rectangular) is a primary quality, while color (being blue) is a secondary quality. So, I have “blue rectangular sensation.” Locke says that my sensation of rectangularity resembles the property in the object (its rectangularity, or its being a rectangle) that produces this sensation in me, but that my sensation of blueness does not resemble the property in the object (its blueness, or its being blue) that produces this sensation in me.

That’s it: that’s the difference: the primary qualities of objects produce ideas in us that resemble those qualities, while the secondary qualities of objects produce ideas in us that do not resemble those qualities. The rest is just a matter of seeing the implications of this, and of understanding why it’s so easy to get confused in trying to make this claim.

A confusion: The following statement is false, but something that many people (including Berkeley) say when trying to explain the difference between primary and secondary qualities:

Secondary qualities exist only in the mind.

Again, this is false, or, at the least, it is not what Locke means or says when he distinguishes between primary and secondary qualities. Remember the first quotation we began with: ideas exist in our minds, and qualities are the powers in objects to produce ideas in our minds. Qualities, that is, by definition, exist in objects, not in our minds.

But why, then, would someone say that secondary qualities exist only in the mind? Recall our example about seeing a blue rectangle. What Locke is saying is that there is nothing in the world that resembles the “blueness” of our sensation. So, one might say, the blue that we see exists only in our mind, and not in the world. But blue, a color, is a secondary quality, so secondary qualities must exist in our minds.

The problem here is that the way we use words such as “blue” tends to blur the distinction between ideas and the corresponding qualities that produce them. Sometime we use the word “blue” to describe a kind of sensation (above, I said that I have a “blue rectangular sensation”), and sometimes we use the same word to describe a quality of an object (above, I said that I perceive a “blue rectangle”). In claiming that my sensation of blue does not resemble the quality that produced that idea in me, Locke is saying that there is nothing in the world that resembles my sensation of blue. It is easy to slip from this to the claim (one that Locke is not making) that there is nothing in the world that is blue, i.e., nothing that has the quality of being blue, or alternately, nothing that has the quality of producing sensations of blue. But this last claim is clearly false, as our experience shows. The claim is not that “blue” exists only in our mind, but rather that, in the case of secondary qualities, what is in our mind (the sensation of blue) does not resemble whatever it is in the world (the quality of blue) that caused us to have this sensation.

Implication:

Secondary qualities are “reducible to,” i.e., can be explained in terms of, the primary qualities of the particles of which objects are ultimately composed. For example, our scientific understanding of how we come to have sensations of blue explains this in terms of the primary qualities of the tiny particles (or wavelengths of light) that enter our bodies. So, secondary qualities are “real” in the sense that objects really do have the powers to produce in us sensations of color, sound, taste, smell, touch, and temperature, but they are not “ultimate” in that our scientific explanations tell us that these sensations are produced in us because of the size, shape, weight, texture, and motion of tiny particles out of which everything is composed. This is what Locke is getting at when he says that the primary qualities of objects “are utterly inseparable from them.” What this means is that primary qualities are the ultimately qualities of the ultimate constituents of matter, and not ones that themselves get “explained away” in terms of more fundamental qualities. Secondary qualities, by contrast, do get “explained away” in this fashion—they are “nothing but” combinations of primary qualities of ultimate particles out of which matter is composed.

I hope this helps!