By a model I understand some familiar kind of object the idea of which we extend by an analogy or metaphor in order to assist our understanding of other things. When we say, “All the world's a stage,” we are using the familiar antics and attitudes of the theatre as a model for thinking about our woes and joys and our activities.

Sometimes a complex model is used by a thinker, by a group, or by a whole age as a guiding pattern suggesting ways of thinking in many areas. One model may even determine an inclusive view about the world from which derive emotional attitudes as well as ruling concepts and ways of thinking for our knowledge of the world. It is often said that the mechanism of a clock served in this way as a ruling model for much thought in the 17th and 18th centuries. Despite their novelty, clockworks were then becoming well known, and they were peculiarly fascinating. Reflection on them made very vivid such notions as regular motion, interchangeable parts, precise mechanical connection, influence by push and pull, and design for specific function.