Three suspects are interviewed about a stolen painting. Arthur says, 'Ben did it.' Ben says, 'Carl did it.' Carl says, 'Ben is lying.' Only one statement among the three is true. Who stole the painting?

Arthur

If Arthur is telling the truth, Ben did it (meaning Ben and Carl are lying). If Ben is telling the truth, Carl did it (meaning Arthur and Carl are lying, but Carl's statement would mean Ben is lying, which contradicts Ben telling the truth). If Carl is telling the truth, Ben is lying, meaning Arthur's statement is false, and Carl's statement ('Ben is lying') is true, meaning Ben didn't do it. Let's test Arthur: If Arthur is telling the truth, Ben is the thief. That makes Ben's statement ('Carl did it') false, and Carl's statement ('Ben is lying') true. This satisfies the condition that exactly one statement is true (Arthur's). Therefore, Ben is the thief? Wait! If Arthur says 'Ben did it', and it's true, Ben is the thief. But Ben said 'Carl did it', which is false. Carl said 'Ben is lying', which is true because Ben lied. That would mean TWO statements are true (Arthur's and Carl's). Let's re-evaluate. Only ONE statement is true. If Carl is true ('Ben is lying'), then Arthur's ('Ben did it') is false, and Ben's ('Carl did it') is false. Thus, nobody is accused by a true statement except Carl's claim that Ben is lying. If Ben didn't do it and Carl didn't do it, Arthur must have done it. If Arthur did it, Arthur's statement ('Ben did it') is false. Ben's statement ('Carl did it') is false. Carl's statement ('Ben is lying') is true (since Ben lied). Thus, exactly one statement (Carl's) is true, making Arthur the thief.