On John Rawls's A Theory of Justice (1971), most of ch. 1-4.
What makes for a just society? Rawls gives us a thought experiment: Imagine you don't know whether you're rich or poor or any of the other specifics of your situation (he calls this going behind "the veil of ignorance" into the "original position"). Now what principles would you pick to determine basic social institutions? Would you choose a caste society where you might be born an untouchable and be screwed? Rawls thinks that in this position you would instead support his basic rules of justice, which are (in short) to make sure everyone has basic liberties, and, more controversially, to allow only such inequalities as bring up the fortunes of those least well off. So you can allow super riches so long as doing so means that the poorest will be less poor than with any other arrangement; the default position is everyone getting an equal share of society's wealth unless you can demonstrate that letting some have more will benefit all.
This theory has been massively influential, and you can easily read it into Obama's speeches. Even many defenders of free-market capitalism do so on basically Rawlsian grounds. The founding fathers (Mark, Seth and Wes) and an especially energetic Dylan debate whether this original position is really coherent and whether it yields the principles that Rawls wants it to.
End song: "Yours to Keep," by Mark Lint & the Madison Lint Ensemble, featuring Bob Linsenmayer. Read about it.
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