Normative behaviourism says that the measure of political principles is how we respond to them in practice, not how they appear to us in theory, but is that a sustainable distinction? Does normative behaviourism end up relying on mentalism, or even utilitarianism? Does it assume too much of the data we either have now or could ever have? Does it bind us to the status quo or presume the end of history? All these are plausible worries, though perhaps not fatal ones, provided one remembers at least two things: first, that we judge this approach by comparing it to the alternatives; second, that we keep on experimenting, both in politics and philosophy alike, including with normative behaviourism itself.