Philosophy of mind
One problem with functionalism is the Chinese room thought experiment by John Searle. Searle tells a story of a man that only speaks English locked inside a room. Inside the room are Chinese symbols and a rule book in English on moving the symbols around. Other people outside communicate with him in Chinese writing. The man can reply in perfect Chinese by following the rules, but does that mean the man knows any Chinese? Searle does not think so, and this means that thinking must happen at a higher level than following rules like a computer.