Supplement to Frege’s Theorem and Foundations for Arithmetic
A More Complex Example
If given the premise that
\(1+2^{2}=5\)
one can prove that
\(1\in \epsilon[\lambda z \, z+2^{2} =5]\)
For it follows from our premise (by \(\lambda\)-Abstraction) that:
\([\lambda z \, z+2^{2}=5]1\)
Independently, by the logic of identity, we know:
\(\epsilon[\lambda z \, z+2^{2}=5] = \epsilon[\lambda z \, z+2^{2}=5]\)
So we may conjoin this fact and the result of \(\lambda\)-Abstraction to produce:
\(\epsilon[\lambda z \, z+2^{2}=5] \eqclose \epsilon[\lambda z \, z+2^{2}=5] \amp [\lambda z \, z+2^{2}=5]1\)
Then, by existential generalization on the concept \([\lambda z \, z+2^{2}=5]\), it follows that:
\(\exists G[\epsilon[\lambda z \, z+2^{2}=5] \eqclose \epsilon G \amp G1]\)
And, finally, by the definition of membership, we obtain:
\(1\in\epsilon[\lambda z \, z+2^{2}=5]\),
which is what we were trying to prove.