Conceptions of professionalism in medicine draw on social contract theory; its strengths and weaknesses play out in how we reason about professionalism. The social contract metaphor may be a heuristic device prompting reflection on social responsibility, and as such is appealing: it encourages reasoning about privilege and responsibility, the broader context and consequences of action, and diverse perspectives on medical practice. However, when this metaphor is elevated to the status of a theory, it has well-known limits: the assumed subject position of contractors engenders blind spots about privilege, not critical reflection; its tendency to dress up the status quo in the trappings of a theoretical agreement may limit social negotiation; its attempted reconciliation of social obligation and self-interest fosters the view that ethics and self-interest should coincide; it sets up false expectations by identifying appearance and reality in morality; and its construal of prima facie duties as conditional misdirects ethical attention in particular situations from current needs to supposed past agreements or reciprocities. Using philosophical ideas as heuristic devices in medical ethics is inevitable, but we should be conscious of their limitations. When they limit the ethical scope of debate, we should seek new metaphors.