Descriptive and predictive functions. Newtonian physics is not a very good tool as a descriptive science, but is fantastic as a predictive science. Quantum mechanics, on the other hand, is a much better descriptive representation of the physical world. Social science, by nature, is less predictive than it is descriptive. Social science 'models' seek to describe processes by which things happen. This is particularly the case in IR. The three main lenses of analysis seek to describe the process by which decisions are made.
(more to come)