In his book Aboutness, Stephen Yablo wants to explain to us when a sentence that is false is nevertheless true about a topic. I suppose ‘all philosophers have long hair’ is false, but it is true about me. Yablo’s construction uses possible worlds, and in particular the idea of a subject matter m which is to be identified with a partition of the set of possible worlds. If m is ‘the colour of grass’, then it is a partition of all the worlds into those where grass is green, those where grass is purple, and so on. According to Yablo’s definition 6 on page 32:

A is true about m in [world] w iff

A is true simpliciter in a world m-equivalent to w

where m-equivalence means that the two worlds are in the same cell of the partition m. The idea is that a sentence is true about a topic in w if there’s a world where it’s true that doesn’t differ from w where the topic is concerned.

This definition works reasonably well for, say, conjunctions. ‘Grass is green and bananas are purple’ is true about the colour of grass, because you can keep the colour of grass green, vary the colour of bananas, and make the sentence true. You don’t have to change anything about grass for the sentence to become true, and so the sentence is true about grass.

But it fails miserably for disjunctions. ‘Grass is red or bananas are purple’ is false in the actual world, but can be made true without changing anything about the colour of grass (by changing something about the colour of bananas). But that means that Yablo’s analysis claims that ‘grass is red or bananas are purple’ is true about the colour of grass. That can’t be right!

Note that on page 33, Yablo tells us that if ‘a statement is true about m in w, it will be used to describe w, because it gets something right.’ But the statement ‘grass is red or bananas are purple’ gets nothing right, even though it is true in our world about the colour of grass (and about the colour of bananas) in Yablo’s definition. I don’t see how to patch this up; it seems to me that the analysis is just on the wrong track.