I've seen a lot of material complaining that members of the "1%" are not paying their fair share toward the costs of running our country. While I'm pretty sure that they are not paying enough, "fair share" seems to be a rather nebulous notion; the obvious response to a complaint based on this meme is a demand for a quantitative set of distribution rules that would be "fair" or unverifiable assertions that the current situation is "fair". Unfortunately, nobody has convincingly proposed a way to determine the correct number. Analogous situations occur all around us; they are difficult to engineer, but they seem to arise quite naturally as the result of evolutionary processes.
What is the "fair share" of the blood supply in a human body for each of the organs that sustain life? It should balance the needs of each organ against those of all of the others to produce a smoothly functioning whole. This balancing is a more or less automatic result of an evolutionary process in bodies. It is very hard to calculate as an engineering exercise. It is easier to recognize when distortions occur that result in disease or failure of the organism.
What happens if some cells in a body decide to grow without limit, and use all of the blood that they can get? This is the situation in various types of cancer. These growths survive and prosper for a time, until their voracious consumption of resources overtaxes the body and death ensues. The analogous situation in our economy is the development of the 1% who take an increasing proportion of all of the money in the country for their use. If the analogy to biological systems holds, this imbalance is not so much "unfair" as it is lethal to the country if not corrected.
Seen in this light, "fair share" is a linguistic device that represents a person's sense of the balanced circulation of money that should supply to each person what is needed to serve the good of the whole country. Elaborating this sense into quantitative rules governing each individual's monetary compensation would be enormously complex, although not more so than trying to determine the ideal circulation of blood within the human body. Casting the argument in terms of "fair shares" is easier, but less compelling. Perhaps the only solution to this problem is a Darwinian process that will lead to our discovering what works by killing societies that don't choose the right distribution when they follow the wrong rules long enough. It would be more desirable (at least from the viewpoints of those societies and individuals who must die in the working of the Darwinian process) to develop, validate, and implement a "theory of distribution" that would enable us to engineer life-supporting policies.