It seems obvious that humans engage in complex thinking some of which is beyond the abilities of other animals. If there are rules of thinking that make us human, we do not seem to be able to describe them. Recent research has claimed to provide a clue to this problem. Studies with human infants demonstrating acquisition of primitive grammar structures that might underlie human language suggested a unique human rule learning ability. This lecture covers some of this research, subsequent research with monkeys along with our recent experiments with rodents suggesting that rats are able to learn similar complex rules and importantly use the rules in new situations. In animals this type of rule learning may support causal understanding.