This paper argues that measurements of opportunity which focus on the contents of a person's opportunity set fail to capture open-ended aspects of opportunity that liberals should value. I propose an alternative conception of which does not require the explicit specification of opportunity sets, and which rests on an understanding of persons as responsible rather than rational agents. I suggest that issues of distributive fairness are best framed in terms of real income, and that meaningful measurements of real income are (...) possible if opportunity as mutual advantage is ensured. (shrink)