Premise: (∃y)(∀x)(Px v Py)
I'm starting out assuming the negation, i.e., (∀y)(~Py).
But then I'm unsure how to find a contradiction within that subderivation. Here's what I've done so far, i know it's wrong because I can't do ∃E
1. (∃y)(∀x)(Px v Py) Premise | 2. (∀y)(~Py) Assume | 3. ~Pa UI 2 | 4. (∃y)(∀x)(Px v Py) Reiterate | b| 5. (∀x)(Px v Pb) Assume | | 6. Pa v Pb UE 5 | | 7. Pb vE 3,6