Intelligence, then, is the ability to attain goals in the face of obstacles by means of decisions based on rational (truth-obeying) rules. The computer scientists Allen Newell and Herbert Simon fleshed this idea out further by noting that intelligence consists of specifying a goal, assessing the current situation to see how it differs from the goal, and applying a set of operations that reduce the difference. Perhaps reassuringly, by this definition human beings, not just aliens, are intelligent. We have desires, and we pursue them using beliefs, which, when all goes well, are at least approximately or probabilistically true.