Although I'm a professor of dogic, I've not had much to say about it so far on this blog. This is partly because I've been busy helping the humans with their strike, and partly because I've been pre-occupied chasing squirrels.
Now that the leaves are returning to the trees, the squirrels are finding it easier to hide, so I have more time to think about my subject. Here I am being thoughtful:
I've been thinking about one of my favourite arguments:
But there's a problem. Imagine a sceptical cat came along (cats tend to be sceptical). The cat says to you "how do you know that it's time for your dinner?" I go through my argument using modus ponens. But the cat says, "fine: but how do you know that the conclusion is true if the premises are true?" I woof that modus ponens is valid. "But how do you know that?" I insist that if the premises are true the conclusion must be true, and that if this holds that form of argument is valid. This does hold, so the form of argument is valid.
"Aha", says the cat, "you've used modus ponens to justify modus ponens: if the conclusion is true when the premises are the argument is valid; the conclusion is true when the premises are, so the argument is valid. But I wanted to know whether modus ponens is an OK form of argument to use, so your justification does nothing to persuade me."
Lewis Carroll, who wrote the Alice books, once told a story about a tortoise which is quite similar to mine about the cat. You can read it here. Myself, I don't think I can answer the cat, and I don't think that I need to. Using modus ponens to reason is just part of what it is to follow the rules for using the word 'if', accepting modus ponens goes hand in hand with understanding 'if'. And as I've said before, I don't think we need to provide a philosophical account of rule-following, for cats or for anything else.
Mental walkies with Lola,