Self-referential definitions can be constructed with the diagonal lemma. Given that the point of the exercise is to show something similar, you’re right that this solution is probably a bit suspect.
I might be wrong, but I believe this is not correct. The diagonal lemma lets you construct a sentence that is logically equivalent to something including its own godel numeral. This is different from having its own godel numeral be part of the syntactic definition.
In particular, the former isn’t recursive. It defines one sentence and then, once that sentence is defined, proves something about a second sentence which includes the godel numeral of the first. But what seed attempted (unless I misunderstood it) was to use the godel numeral ┌ψ┐ in the syntactic definition for ψ, which doesn’t make sense because ┌ψ┐ is not defined until ψ is.
Don’t know if this is still relevant, but on Ex9
you definitely can’t define ψ this way. Your definition includes the godel numeral for ψ, which makes the definition depend on itself.
Self-referential definitions can be constructed with the diagonal lemma. Given that the point of the exercise is to show something similar, you’re right that this solution is probably a bit suspect.
I might be wrong, but I believe this is not correct. The diagonal lemma lets you construct a sentence that is logically equivalent to something including its own godel numeral. This is different from having its own godel numeral be part of the syntactic definition.
In particular, the former isn’t recursive. It defines one sentence and then, once that sentence is defined, proves something about a second sentence which includes the godel numeral of the first. But what seed attempted (unless I misunderstood it) was to use the godel numeral ┌ψ┐ in the syntactic definition for ψ, which doesn’t make sense because ┌ψ┐ is not defined until ψ is.