Arguments about the desirability of this or that taxation measure or policy invariably involve assertions about their incentive effects on human behaviour. Will it lead people to making different choices? Are these choices better or worse than those that would otherwise have been made? The first of these questions is empirical; the second can only be answered in relation to whatever is postulated as the preferred set of desirable outcomes, and such postulates are essentially contestable. Inevitably, arguing about them leads us to more general tenets about social goods. Arguments about these tenets are, in the end, ultimately philosophical. In the course of these arguments, ethically laden terms like equity, equality, justice, fairness, and moral obligation shift from the wings to the centre of the stage on which the argument is conducted.