   In an intuitive sense, ‘fitness’ can refer to the correspondence between the shape of an object and an empty volume it is placed in: a square peg fits into a square hole. In a more abstract fashion, one might refer to the properties of an entity and how they correspond to the constraints of its context. In a biological setting one could focus on an organism’s traits and how they correspond to various aspects of the environment the organism is living in. Following biological usage, call this concept ‘ecological fitness’. (It has also been called ‘vernacular fitness,’ cf. Matthen and Ariew 2002.) The ‘vernacular’ definition is fraught with difficulties. Suppose, following Dennett (1995) we characterize the relation ‘\(x\) is fitter than \(y\)’ as follows: