You should write down three statements using the symbols \(P, Q, R, S\text{.}\) If Geoff is a truth-teller, then all three statements would be true. If he was a liar, then all three statements would be false. But in either case, we don't yet know whether the four atomic statements are true or false, since he hasn't said them by themselves.

A truth table might help, although is probably not entirely necessary.