When does ‘Can’ imply ‘Ought’?

The Assistance Principle is common currency to a wide range of moral theories. Roughly, this principle states: if you can fulfil important interests, at not too high a cost, then you have a moral duty to do so. I argue that, in determining whether the ‘not too high a cost’ clause of this principle is met, we must consider three distinct costs: ‘agent-relative costs’, ‘recipient-relative costs’ and ‘ideal-relative costs’.