It is important to identify the easy cases of update. To this end, McCarthy has developed a notion of Cartesian counterfactuals. A use for these formal objects, is as a way to specify how a knowledge base should change in light of new information. These counterfactuals are not as powerful or expressive as the philosophers' models, but they have the distinct advantage that they allow fast, efficient update. They are based around the idea that a basis, in terms of axioms, or structural properties of a knowledge base can be given. Update is then carried out relative to this basis, analogously to Cartesian co-ordinates.