For us, responsible people, it is clear that just and fair is to be rewarded accordingly to our contribution. If we try harder, work more and better and create more value as a result, we expect to get more in return. And accordingly, if we don’t try or we do a lousy job for whatever reason, we understand that we deserve less for it. An abominable contrast to it is the altruistic system that commands that we shall not ask more for doing a good job. In fact we shall not ask for any rewards at all. The rewards go to those in need, regardless of their contribution or how deserving they are. This system is fundamentally unjust.

While this is clear, many people hold a different view that leads to a very common conflict – and not only among philosophers. A typical objection I keep hearing is the following:

Different people have different opportunities that they cannot affect. Why should that make some people better off than others? Imagine two identical children. One is born into a rich family that provides it with good education and raises it to be confident and successful. The other’s parents are poor and abusive, the child receives little education and grows up to be a nerve-wreck. Both of them start to work and put equal effort into it, doing the best they can. Is it fair that the first receives vastly higher wage and acclaim?
The person telling you this believes that it is fair to reward people according to their effort and not the actual value their work creates.

I have to admit that this does make sense in a way. In line of the ethics we started off with, a person should be rewarded based on what they do. So why should a person be punished or rewarded based on things that are not in their power to change? Rewarding people based on their effort indeed is fair as well. Another perspective comes from the negative side. While I abhor the idea of a poor lazy person getting someone else’s money in welfare, I similarly dislike it when an arrogant moron makes a lot of money just because he was born with a golden spoon in his mouth. Formally, he may create a lot of value, but only a tiny fraction of it can be really credited to him. I find them both undeserving.

How can that be? How can there be two conflicting definitions of a just reward at the same time?
As always – “Whenever you think you are facing a contradiction, check your premises”.
Hint comes when you need to give an answer to the person with their heartbreaking children story. Maybe you have a better one but the best answer I can come up with is – “The world is not fair. It sometimes sucks, but we just have to live the life the best we can with the cards we are dealt.” Which is a lousy answer when trying to explain what fair means.

The reason is that these two cases of justice, while talking about the same thing, are based in different worlds.
Those different starting conditions that our objector complained about are based in the real world. It is the reality we all live in, which is without values or feelings. It deals to everybody, everyday, something different, and there is no fairness in it whatsoever. It is what it is.
On the other hand, the justice we wanted originally applies on a higher, abstract level – on the level of us humans and our ethics. It is the level where values do exist and the one that we can choose and change. So while the conditions each of us is given are unfair, we can create fairness in how we interact.

While this explains how two seemingly colliding definitions of fairness can coexist, it does not say how to deal with the violation of the later. Unfortunately, dealing with unfair conditions is an open problem and previous attempts to solve it have led to some top-tier catastrophes. Formally, the statement of both is quite clear. But the practicality of their solutions differ widely.
The problem is that the amount of “value” we can distribute is limited. We only have as much as we create. Value can’t be drawn out of thin air – regardless politicians often saying otherwise.
Giving a fair reward proportionally to the value created is straightforward. It just means exchanging value for value in a corresponding manner. Value created is distributed back proportionally and without significant issues. So the overall vision that 1) the world is not fair, deal with it; 2) but reward everybody according to their contribution – is simple, clear, consistent, and easy to implement.
On the other hand, trying to fix the unfairness of the world itself and reward people according to their effort is impractical. There is no way to objectively assess an effort a person is making. If somebody creates something of value to you, you don’t need to care how and why they did it – the value, for you, is objective. But knowing how hard they tried? Was it an incapable person doing their best, or a very capable one but slacking, or a one specializing in the skill of acting out a hard effort?
While we can make a personal call and pay extra to a person we know to be good and honest and trying hard even though they did not do so much at the end, this can’t practically be extended to a large scale. Any attempt to do so inevitably fails on the subjective nature of an effort. Moreover, since it can’t be correctly assessed, it only creates wrong incentives for people – to pretend to try, instead of doing actual good work – destroying value for everybody as a result.

“World is not fair” is a poor answer, but currently the best we have. Trying to fix that, on a global scale, should be done with utmost caution as such attempts have already cost hundreds of millions of lives. Until somebody figures something out (naive wishful thinking really does not count), we should stay content with playing the cards we were dealt and the rewards we deserve. Which is not that bad.