Propositions generally seem to be about things. The proposition that Tibbles is on the mat is about Tibbles and a mat. The proposition that 2+2=4 is about some numbers. The proposition that chips are on the counter is about chips and a counter. And so on.
Some propositions (statements, sentences, beliefs, etc.) are intuitively about themselves. For example, the proposition that all propositions merit investigation is intuitively about itself.
Now for a paradox. Consider the following proposition P: every proposition that is not about itself is mundane.
P is paradoxical because it seems to be about itself if and only if it is not. Let me draw this out. Suppose first that P is about itself. Then we can show that P is not about itself as follows. P is about all and only those propositions that are not about themselves, for it says that each is mundane. So, P is not about any proposition that is about itself. Therefore, P is not about P if P is indeed a proposition that is about itself. Therefore, P is not about itself if it is about itself. Suppose, on the other hand, that P is not about itself. We have already observed that P is about those propositions that are not about themselves (because P reports that each is mundane). Therefore, if P is one of those propositions that aren't about themselves, then P is about P. So, either way, we fall into contradiction.
We can make the paradox more acute by stipulating that 'x is about y' means 'x quantifies over instances of a kind of which y is an instance'. We can then ask whether or not P is about P in that precise sense. (If you think you see a way out of the paradox, ask yourself if there's a way to re-write the paradox that avoids your solution, and I'm guessing you'll see that there is.)
This paradox will remind you of Russell's paradox concerning the set of all sets that aren't members of themselves. But I believe the paradox of propositions is much harder to solve. Concerning sets, we can, if we like, treat "set" talk as plural reference talk (thereby eliminating the existence of sets altogether), or else we may carefully craft axioms of sethood (such as ZFC) that preclude the existence of sets that are members of themselves.
But such solutions are not nearly as promising when it comes to propositions. If you think we can simply eliminate propositions, then run the paradox in terms of sentence tokens: the sentence token represented by P surely exists (or at least there are things arranged P-wise...). We might try to craft axioms of aboutness to get out of this, but such axioms won't take away the deep feeling that P should be about itself if and only if it is not. (With sets, by contrast, there is something right about supposing that no sets contain themselves.) So, we have a paradox on our hands that appears to be more serious than previous ones of its kind.
The paradox could perhaps be viewed as evidence against the reliability of our a priori faculties (though, of course, we'd have to rely on those same faculties to "see" this!) Or, more drastically, someone could view it as evidence that reality is at bottom absurd. I think it should be viewed as an invitation to gain a deeper understanding of the nature of propositions and aboutness.
Suggested ways of resolving the paradox are welcome. (I have a solution, but before I share it, I'd like to see how others might solve the problem.)
