If A implies B, then is there always something that we can point to as what A adds to B? The logician, or logical engineer, says yes. The mysterian says no. To get a bead on the issue, this chapter distinguished four types of extrapolation: inductive, as in Hume, projective, as in Goodman, alethic, as in Kripkenstein, and type 4, as in Wittgenstein's “conceptual problem of other minds” and his example of 5 o'clock on the sun. Logical subtraction is understood, to begin with, as type 4 extrapolation. A–B is the result of extrapolating A beyond the bounds imposed by B. The question is whether this can always be done.