There is an old joke that goes something like this: “If God is love, love is blind, and Ray Charles is blind, then Ray Charles is God.” Explain, in the terms of first-order logic and predicate calculus, why this reasoning is incorrect. If the person who told you this joke was your grandmother, how would you respectfully explain this to her?

Question

anonymous · Answer

Note that we have the case that God is love, and love is blind, therefore God is blind is a correct conclusion from our principles. But, the case that Ray Charles is blind, and therefore he is God is not a correct conclusion, as we are taking an implicative (forward) conclusion, and reversing it. In essence, what we are doing is that $P \implies Q$ is true, but it is not necessary that $Q\implies P$ is the case.

anonymous · Answer

I think you set: God is love = P
                      Love is blind=Q
              Charles is blind    = R
         Charles is God          = S
 and then, make a statement like, (P^Q)^ R ---> S
 construct a truth table to get a mathematical logic of a wrong statement/