We have approached human behaviour from a number of angles, some of them familiar, others relatively novel. Causality seems to be an inevitable theme, whether we discuss the relations between an agent and his deed, the contrast between doing and undergoing, explanations of behaviour, trying and doing, or intention. The next topic I shall consider, what it is to be able to carry out one's intention, to try or to succeed in an attempt, must seem excessively familiar. And sure enough, causality will turn up again. Philosophers have raised such questions as whether one's ability might be a cause of one's performance; whether it means the same to say 'John can do X' and 'If John wants (decides, tries) to do X, that will cause him to do X'; and whether it is consistent to assert both, 'Something caused John to do X', and 'Nevertheless he could have done something other than X.'