The classical theory of computability attempts to characterize the power of perfect computers…that do not share the practical limitations of real machines. The usefulness of these models can be compared to that of the equations of free fall or the orbit of a planet. Just as no object falls according to these equations due to frictional forces or other forces that need to be neglected to make the problems tractable, these models also serve as idealized models of computational devices whose properties can be approximated to within any degree of accuracy by real machines.