From Creatures Wikia
As Steve Grand once put it:
- There's much more to intelligence than logic. Most of the AI pioneers were mathematical magicians and philosophers and to them, thinking was about logic, about reason. But it has to grow out of more primitive systems, and most intelligence is not logical. Most of the time we are not reasoning people. Dogs tend not to argue about syllogisms, but they still seem bright. Chuck IBM's Deep Blue chess computer and a dog into a pond, and see which one climbs out first. Which means that intelligence is grounded in survival. If you haven't got a reason to think, you won't think, and survival is what motivates us. 
For more information, see the Wikipedia article on Artificial life.