  But can we do no better? The present author (1985) proposed to set aside the problem until more is said about various possible constraints on the solution. Unless we had some reason to think that such functions really did describe the world, we might well feel that a solution was less than imperative and less than unique. For example, the world might be described wholly with \(C^{\infty}\) functions \((n\)-th derivatives exist for all \(n\), e.g., cos, sin, log, exponential functions). The above function is not among these, since its derivative is discontinuous. But then it isn’t clear what we might say of it if the example is counterfactual. There might be different things to say depending on what further principles describe the possible world. Hence we would need to supplement the original statement of the problem with an argument to the effect that we might expect such functions to describe the real world, or alternatively supply additional metaphysical principles to be regarded as constraints on the solution.