Paul Krugman and Ambrose Evans-Pritchard have been jousting on the question of whether the central bank can create inflation at the zero bound or not. I don't think either of them have teased out all the subtleties, and, partly as a consequence, are talking somewhat at cross-purposes. In particular, we need to clarify whether we are talking about a permanent or a temporary zero bound episode; and whether we are talking about conventional open market operations or a helicopter drop. And we also need to clarify whether we are conjecturing what happens in the real world, or just talking through the properties of a model.