What is a relational property? In an earlier question about a car driving down a road and appearing to get smaller with distance, Prof. Moore wrote that this appearance is a relational property of the car, as opposed to the real size of the car, which is an intrinsic property of the car --- and what I see is this relational property. [See, http://www.amherst.edu/askphilosophers/question/548.] But it is clear to me that what I see is a small car: how can a small car be a relational property, whatever that is?

A relational property is a property a thing has only in virtue of how it is related to something else. A common example is fatherhood. Whether I'm a father depends upon my relation to something else, namely, my child (if I have one). So the claim is that, in so far as the car appears to be small, its apparent size is merely a relational property. It is, for example, a matter of how large a portion of your visual field the car occupies.

Moore's point was that it's not the car itself whose size changes as it recedes, but only how large a portion of your visual field the car occupies. Now, it's certainly true that one is given to describing this phenomenon by saying that the car gets smaller. But, of course, one doesn't really think the car itself gets smaller, and there is an obvious sense in which it seems to remain the same size but to get farther away. To suppose there is some contradiction here is to suppose that how large an object appears to be is a direct function of how much of the visual field it seems to occupy, and that's simply not true. (There are well-known visual illusions that demonstrate the principles at work here.)

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