(From a Facebook discussion about income inequality:) As I get older I get less and less enamored with using income as any kind of proxy measure of net human contribution, and by extension, of "fairness." Income is determined by the vicissitudes of whatever the marketplace happens to value at the moment, and markets are notoriously prone to short-term thinking and to encouraging participants to burden others with their "externalities," that is, with their side-effect costs such as emission of air- and water pollution. (Socialize the costs, privatize the profits.)
EXAMPLE: For several years I've volunteered a few hours a year in the Real Men Read program, reading "chapter books" to underprivileged elementary-school kids (I was recruited by a close family friend who's a teacher). I can easily argue the case that in the long term, no one is more important to society than school teachers. Yet we pay even the best teacher a tiny fraction of, say, the income that Martin Shkreli has managed to capture. The marketplace has highly valued his skills and has rewarded him accordingly. But his income is in no way a valid measure of his long-term contribution as a human being and thus of what's "fair" for him to get.
(In fairness to Skreli, I think he might well be mentally ill — or at least he's putting on a good show of it for his criminal trial — but my point remains.)