Mental life [is] something we [know] very little about, and when something [is] imprefectly understood it [is] quite likely we [will] define its structure imperfectly too. It [is] only rarely that, in science, you [start] with a perfectly delimited thing and set out to investigate it; more often, your definition of what it [is] you are looking for [changes] as you [discover] more about. "Consider the medieval physicists who wondered what fire could be...They identified a range of things they thought were instances of fire: burning wood, the sun, comets, lightning, fireflies, the northern lights. They couldn't give a definition, but they could give examples that they agreed upon. [But] the category fire, as defined by what seemed to be intuitively obvious members of the category, has come completely unstuck. Turns out that burning wood is actually oxidation; what happens in the sun has nothing to do with that, it's nuclear fusion; lightning is thermal emission; fireflies are biophosphorescence; northern lights are spectral emission".-- from Patricia Churchland quoted in "Two Heads - a marriage devoted to the mind-body problem", by Larissa MacFarquhar.