Full Idea
There is nothing arbitrary about the prima facie duties; each rests on a definite circumstance which cannot seriously be held to be without moral significance.
Clarification
'Prima facie' means 'at first glance'
Gist of Idea
Prima facie duties rest self-evidently on particular circumstance
Source
W. David Ross (The Right and the Good [1930], §II)
Book Reference
Ross,W.David: 'The Right and the Good' [OUP 1930], p.20
A Reaction
He goes on to list the duties. Some of these duties will inevitably arise if we acknowledge both the rightness of keeping contracts, and the desirability of increasing general happiness.