You and I share a well. We both thought it would never run dry. I used a lot more water than you did. Now we discover it is going dry. We need to proceed with care, limiting our use. The well reaches an underground lake that is fed by springs. We have been drawing at a rate that outpaces how fast the springs replenish it. So be it. But who will get how much? I make the following proposal: let each have an equal share going forward, be it per household or per person. You object. You have used a lot less than me over the years. You think that should be part of the accounting. "Yes," you say, "let there be a fair share, but looking back as well as forward." But until now, neither of us knew the well might run dry. The supply seemed inexhaustible. So my using more than you did not seem to matter to either of us. The idea of a "fair share" makes no sense if what is to be shared is inexhaustible. But it wasn't inexhaustible, we just thought it was. Should ignorance matter here? Does it make any difference? True, ignorance is said to be no excuse in the law. But in the case of the law these is something to be known that it was my responsibility to learn about. Here we had an unknown. Even though neither of us realized it, I got an unfair advantage. Unfair or just lucky? Suppose we say it was luck. I happened to be in the right place at the right time to make use of more of the water than you. Does that luck count in the moral equation of who should get what going forward?
We like to think we should only be held to account for what we have control over. And how can I have control over that which I am ignorant of and could not even have known if I had wanted to? (Although when you and I do the same thing and by pure luck, your act has consequences that mine does not, things get complicated. A child runs in front of your car, not mine. We were both driving with reasonable vigilance. For better or worse, you killed the child. It seems extreme to say that you have no moral responsibility at all. And yet your act and mine were identical.) But here we are not worrying about moral culpability but a fair distribution. How we may feel about this may turn out to be a function of the detail:
• You and I share a well which we assume has an unlimited supply of water. I used more than you. I like children, you don't. I used the water to grow crops to feed my children. Now a fair share that looks backward as well as forward will not yield enough for my family. Our plans we paid in good faith and upending them will have dire consequences. That seems unfair.
• You and I share a well which we assume has an unlimited supply of water. I used more than you. I had a swimming pool and fountains; you did not. A fair share looking forward alone will leave my pool empty - so be it. But a fair share looking backwards will hurt more than that. As long as I am guaranteed some supply for my basic needs, that does not seem unfair. I enjoyed my pool. Now it's your turn.
What is the difference? In both cases, if I had known the truth about the water supply I might have acted differently - had fewer children or not built the pool. But what is done is done. Yet in one case, the plans I laid can be aborted. The pool just stands empty. The children are another matter.
• You and I share a well which we assume has an unlimited supply of water. You used less than me. Your interests lay elsewhere. You could have used more. You simply chose not to. Here including a backward looking claim seems gratuitous.
• You and I share a well which we assume has an unlimited supply of water. You used less than me. You wanted to do more, but you lacked the money to invest in irrigation equipment to extend your fields. Now you have the funds to invest and you want to make up for lost time.
Of course here the difference is that in the second case you had a plan, albeit a frustrated plan. In the first you simply squandered an opportunity. I benefited from your situation in both cases.
If you share my intuitions in these cases, they show how luck is not dispositive. When it comes to a fair distribution, whether history matters or not may depend on the details.