Making Life Hard For First-Order Logic
Posted by David Corfield
I mentioned in a previous post Sundholm’s rendition in dependent type theory of the Donkey sentence:
Every farmer who owns a donkey beats it.
For those who find this unnatural, I offered
Anyone who owns a gun should register it.
The idea then is that sentences of the form
Every that s a , s it,
are rendered in type theory as
where and are the projections to first and second components. We see ‘it’ corresponds to .
This got me wondering if we could make life even harder for the advocate of first-order logic – let’s give them the typed version to be generous – by constructing a natural language sentence which it would be even more awkward to formalise.
One thing to note is that the type above has been formed from a dependency structure of the kind:
Now I’ve been writing up on the nLab some of what Mike told us about 3-theories back here. Note in particular his comments on the limited dependencies which may be expressed in the ‘first-order 3-theory’:
one layer of dependency: there is one layer of types which cannot depend on anything, and then there is a second layer of types (“propositions” in the logical reading) which can depend on types in the first layer, but not on each other.
So to stretch this 3-theory we should look for a maximally dependent structure. Take something of the form
Ideally the types will all be sets.
By the time quantification has been applied, the left hand side swept up into an iterated dependent sum and a dependent product formed, the proposition should be bristling with awkward anaphoric pronouns.
The best I could come up with was:
Whenever someone’s child speaks politely to them, they should praise them for it.
A gendered version with, say, men and daughters would help with the they/them clashes.
Whenever a man’s daughter speaks politely to him, he should praise her for it.
Any better candidates?
Re: Making Life Hard For First-Order Logic
It seems the first appearance of a donkey sentence was in 1328!
Only six and a half centuries later we had a proper formalism for it.