This post is primarily for discussion of the Primary-Secondary Quality distinction, though of course spin-off discussion is welcome (and bound to occur)!
I'll go ahead and start things off, with a brief description of primary and secondary qualities.
Primary qualities are properties of an object that can be characterized--nearly enough--in mathematical terms, particularly geometrical terms: length, depth, breadth, shape, location, and motion. An object possesses its primary qualities independently of its relations to other objects. In a word, primary qualities are intrinsic qualities.
An object's primary qualities are also objective in roughly Nagel's sense of "objective". That is, objects possess their primary qualities independently of how anyone or anything experiences the object.
Secondary qualities are different. Unlike primary qualities, an object possesses its secondary qualities only insofar as the object is related to other objects. Secondary qualities are thus relational qualities. No object possesses relational properties entirely on its own--other things have to exist. Consider the property of being a brother. Nothing is intrinsically a brother. Being a brother requires that something else exist. So, the property of being a brother is a relational property. Secondary qualities are a bit like the property of being a brother.
Secondary qualities are in fact a special kind of relational quality: they are causal-dispositional qualities. That is, they are qualities an object possesses in virtue of how it affects (or is disposed to affect) other objects. Consider the property of being dangerous. Something is not dangerous in and of itself. (Consider: it does not seem to make sense to suppose that an object could be dangerous if it were the only thing in the entire universe.) Rather, being dangerous is a matter of a thing's being disposed to cause harm to other things (or something like that).
According to Locke (and Galileo and Descartes and many others), color, sound, scent, taste, warmth and cold, and the like are--insofar as they are properties of objects at all--are secondary qualities. They are properties that objects have insofar as they cause (or are disposed to cause) effects in other things.
Actually, we can be yet more specific: these secondary qualities are mind dependent properties. Objects possess color, taste, warmth and cold, in virtue of how they (are disposed to) cause ideas in the minds of perceivers.
For instance, an object is red if and only if it is disposed to cause the idea of red in the minds of certain beings. An object is warm if and only if it causes (or is disposed to cause) the idea of warmth in the minds of certain beings. And so forth.
Now, note that secondary qualities are related to--and dependent upon--an object's primary qualities. For an object will be disposed to cause certain ideas in the minds of perceiving beings only if it possesses the appropriate primary qualities (intrinsic structure). To give a terribly oversimplified example: an object will reflect light differently if its surface "texture" is different. Of course, it will also depend on the nature of the perceiver's mind. But this isn't surprising--secondary qualities are relational properties, after all.
Note that, according to this view of the secondary qualities, objects do not possess them (color, warmth, etc.) in and of themselves. They do possess them--but only in virtue of how they are related (how they are disposed to affect) other objects. Indeed, they are subjective properties in they sense that they depend for their existence upon the existence of subjects of experience (perceiving beings).
So that's the primary-secondary quality distinction. It was widely accepted by early modern philosophers (with some exceptions--Bishop George Berkeley being perhaps the most prominent), and indeed it is widely accepted by scientists today.
But all this is only half the story. Things get a bit more complicated when we bring in the ideas of primary qualities and ideas of secondary qualities. Hm. But I think this is enough for now, don't you? We can talk about the ideas of primary and secondary qualities in another post.
I look forward to your comments on this topic! Does this make sense? What would you like clarified further?
Of course, we've already had one question, and I want to address it here. Olivia asked--and Laura reiterated--the following (good) question: Might it not be the case that the distinction between primary and secondary qualities is a kind of illusion? Perhaps it results from a kind of accident about the way we experience objects. Perhaps we think certain properties (like shape) are primary because we experience them as invariant properties of objects, secondary qualities as variable. But if so, perhaps we simply haven't had the sorts of experiences with objects that would reveal the former properties as variable.
This is a Berkeleyan strategy, insofar as it disputes the distinction between primary and secondary qualities. It isn't Berkeley's way of challenging the distinction though. Perhaps we can get to that later. But let me attempt to respond to this challenge.
The first thing to observe is that the primary-secondary quality distinction is not explained in terms of invariance/variability. The distinction is, rather, drawn in terms of the distinction between intrinsic properties (those an object has independently of anything else) and relational properties (those a thing has that depend on the existence of other objects). These seem to be different distinctions, and if so, the objection doesn't serve to show that there isn't really a distinction between primary and secondary qualities.
Now, the objection might still have force if it turns out that our belief that a property is intrinsic depends on our experiencing it as an invariant property. But I'm not convinced this is true. After all, it is easy to imagine an object that changes its shape every few minutes even if it is the only object in the universe. But then, since there is nothing else, it has a variable, intrinsic property. Moreover, I think we often treat variable properties (like the changing shape of an inflatable balloon) as intrinsic--not dependent on how it affects our minds or indeed on its relations to anything else.
I think there is one other point worth making here: and this is that I'm not sure it makes sense to suppose that all of an object's properties could be relational (secondary). Relational properties, it seems, have to be "grounded" in non-relational (intrinsic, primary) properties. For now, I'm going to leave this claim unargued for. It sounds like a perfect topic for further discussion after all!
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