What is the Problem of Simplicity?

The problem of simplicity involves three questions: How is the simplicity of a hypothesis to be measured? How is the use of simplicity as a guide to hypothesis choice to be justified? And how is simplicity related to other desirable features of hypotheses -that is, how is simplicity to be traded-off? The present paper explores these three questions, from a variety of viewpoints, including Bayesianism, likelihoodism, and the framework of predictive accuracy formulated by Akaike (1973). It may turn out that simplicity has no global justification -that its justification varies from problem to problem. Scientists sometimes choose between rival hypotheses on the basis of their simplicity. Nonscientists do the same thing; this is no surprise, given that the methods used in science often reflect patterns of reasoning that are at work in everyday life. When people choose the simpler of two theories, this “choosing” can mean different things. The simpler theory may be chosen because it is aesthetically more pleasing, because it is easier to understand or remember, or because it is easier to test. However, when philosophers talk about the “problem of simplicity,” they usually are thinking about another sort of choosing. The idea is that choosing the simpler