I want to explore the possibility of an a posteriori approach to the elucidation of certain moral notions. These are: (a) the notion of a duty, some specific thing which it is incumbent on me to do, and (b) the notion of something that is a good thing for me to do. I want to consider these notions, so far as I can, independently of rules. There is a certain sense in which having a duty to do this or that is a function of circumstances, and in which this or that's being a good thing to do is likewise a function of circumstances. I shall suggest specific examples in which this is a conspicuous feature of ‘my duty’ or of what I can, beneficially, do. In these examples what I ought to do, and what it is good to do, can be represented as special ways in which what I am to do presents itself.