The problem with ethics in most descriptions and treatments, from Aristotle, through Locke, and since Rawls, Nozick, and Parfit is that the sorts of questions we ask, such as “what is the good?” or “should we do X?” all seem to rely on a conception of the surrounding environment as wholly unlimited in scope and resources. We take it for granted that if there is a good to be had through acquisition, then all we need ask is “who came first?” without considering what to be said of the man who comes last to the table, when all else has already been allocated.
I'm not trying to suggest that resource constraints are ignored. The realities of socialized housing, healthcare, and sustenance make the ethics of resource allocation unavoidable. What I want to point out though is that of all the grand works of ethics we currently have to rely on, and discuss the individual merits of - liberalism vs. egalitarianism, collective consequentialism, and so on - are all built out of an abstract conception of what man and his rights consist in, and only then from this conception does the ethicist attempt to engage with the hard issues of limits on growth.
For instance, Locke builds his system of values from the notion that a man holds the same rights over himself as he does any of his property; Rawls begins from a similar egalitarian view, where the talents or deficiencies of the individuals in a group are unknown, thus the best way to achieve the maximal good for all is to distribute resources in a fair manner.
But life, as we know, is not fair. There may easily come a point in our future where the vital necessities of life are neither as plentiful as they are now nor as available. Perhaps we’ll invent a new need, or new benefit, which can only be filled for a billion- not seven. I cannot bring myself to accept a theory of ethics that relies on the blind fortune of lottery to achieve “fairness,” because this seems the exact opposite of fair - judging a person worthy of a benefit over his neighbor on the basis of an arbitrary rule.
This is why I believe that to begin constructing ethics, if we want to begin from a conception of human right, then we need to start not from the nature of man in a vacuum, but of the nature of men in conflict. Ethics, by and large, is not there to prescribe how we ought to live when we are alone, but how we ought to conduct ourselves in dealing with others. It is primarily a means of resolving our interpersonal conflicts by appeal to Reason, rather than by an appeal to the whims of God, Men, or dice.
So how could we begin otherwise than by starting from the essential conflict of life? Here's the scenario I want to begin with, and which we can easily imagine: A woman in the desert comes upon two men, alone, fit, but thirsty. She has the last bottle of water for miles, and knows the way to her home where everyone may survive.
There is only enough water for two to make the trip. Whoever is left without water will perish, horribly, from thirst and baked by the sun.
What's the ethical thing to do?