The "validity" such an argument has(if that is the right word) is presumptive and provisional in nature.5 It is frail, andsubject to default.Even so, such presumptively based arguments can be very useful and important in cases where action must be taken, but firm evidence is not presently available. Examples would be in planning, where the future holds many uncertainties,or in practical deliberation, where prudent action often requires acting on provisional hunches and guesswork, always subject to revision, as better informationcomes in.