  The second generalist line of defence involves drawing in one’s horns a little. Ross distinguishes between derived and underived prima facie duties. The underived ones are the duty to do the just thing, to act for the best, not to cause harm, to keep promises, and so on. Other duties are derived from these. So there is, as we might put it, a core of invariability surrounded by a variable periphery. I might have a duty to go up to London today to see my son Hugh. But this duty is derived from a general duty to do what I have promised to do. As we might put it, that Hugh is expecting to see me today sometimes gives me a reason to go up to London and sometimes does not; it is a derived, and therefore variable, reason. If it does give me a reason, it will because it is keyed in some way into an unvariable, underived reason.  So derived reasons are variable, and underived ones invariant. On this account, counter-examples will only do damage if they are aimed at the supposed underived reasons. (See McNaughton and Rawling 2000.)