In science, an explanation for observational data. A good scientific theory explains as much of the data as possible (a perfect theory would explain all of it), is as elegant as possible, and requires as few miracles as possible. A bad scientific theory conflicts with the data, is less elegant than another theory attempting to explain the same data, or requires miracles.

In philosophy, a set of propositions and their closure under logical implication, used to present a view about some philosophical issue. A good philosophical theory is consistent, is elegant and requires few miracles, but ordinarily does not need to explain any observational data. (Sometimes, however, philosophical theories are constructed in order to explain certain intuitions people have about notions such as free will, meaning, or the world before they begin to think in philosophical terms.) A bad philosophical theory often is inconsistent, requires an inelegant ontology or is question-begging.