Defining theoretical terms

This article investigates David Lewis's influential account of the implicit definition of theoretical terms, especially his curious, often overlooked use of non-classical logic. It is explained why Lewis opted for the positive free logic of descriptions FD2, which implausibly renders all identities between empty terms true. And it is argued that Lewis's account is best implemented in the weaker free logic of descriptions MFD, with classical second-order logic as an alternative. The philosophical core of Lewis's account is also clarified and motivated, including the controversial view that true term-introducing theories are uniquely realized.