I'm having some doubt if this proof is valid or invalid, especially regarding the
line 4 derived from
line 2. Do I need to change the letter in there?
1. (z)~Fz ∴ ~(z)Fz Using Indirect proof strategy we assume the opposite i.e., 2. (z)Fz 3. ∴ ~Fa [from 1; Dropping universal] 4. ∴ Fa [from 2; Dropping universal] 5. ∴ ~(z)Fz [from 2]
And the rule is that if we have a contradiction, our proof is done. Is there any way to check this if it is valid or not?