Suppose someone says “It’s cold outside!” and you want to know if they mean it’s below 40 degrees rather than below 60 degrees. One way to tell what they mean is to put them in 40 degree weather and witness them say “It’s cold outside”, then put them in 60 degree weather and witness them say “It’s not cold outside!“.
Now let’s abstract that: a straightforward way to tell if someone saying means rather than is to show them ’s (and witness them say ) and then show them ’s (and witness them not say ). Call this witnessing the distinction via dispositions of the -speaker. Disjunctivitis is a thought experiment that says this is not a sufficient tool for us to distinguish ’s and ’s generally. 1
We’re worried about whether the word ‘porcupine’ really means porcupine, or whether it means porcupine or echidna. Because most of us porcupine users can’t tell porcupines from Australian echidnas. And, although everything I’ve ever seen in called a ‘porcupine’ was a porcupine, everything I’ve ever seen called a ‘porcupine’ was also a porcupine or echidna. And the question is, well, in virtue of what fact is it ‘porcupine’ actually means porcupine, and not porcupine or echidna?
You can’t tell it from my dispositions on the output side. On the input side, although it might have been equally true that they’ve all been porcupines, they’ve also been porcupines or echidnas. (Furthermore, for all I know, they all happened to have been been male porcupines, in which case we have to worry that my word ‘porcupine’ means male porcupines, and, again, I can’t tell them from the female ones.)
Solution from Sellars: the people who taught you the word ‘porcupine’, what pattern were they trying to inculcate in you? Was it to say ‘porcupine’ when confronted with a porcupine or echidna? This is a crucial piece of how the noise ‘porcupine’ coming out of my mouth means porcupine.