Suppose the true source of income was a gigantic pile of money meant to be shared equally amongst Americans. The reason some people have more money than others is because they got to the pile first and greedily took an unfair share. That being the case, justice requires that the rich give something back, and if they won't do so voluntarily, Congress should confiscate their ill-gotten gains and return them to their rightful owners.
A competing liberal implied assumption about the sources of income is that income is distributed, as in distribution of income. There might be a dealer of dollars. The reason why some people have more dollars than others is because the dollar dealer is a racist, a sexist, a multinationalist or a conservative. The only right thing to do, for those to whom the dollar dealer unfairly dealt too many dollars, is to give back their ill-gotten gains. If they refuse to do so, then it's the job of Congress to use their agents at the IRS to confiscate their ill-gotten gains and return them to their rightful owners. In a word, there must be a re-dealing of the dollars or what some people call income redistribution.
The sane among us recognize that in a free society, income is neither taken nor distributed; for the most part, it is earned. Income is earned by pleasing one's fellow man. The greater one's ability to please his fellow man, the greater is his claim on what his fellow man produces. Those claims are represented by the number of dollars received from his fellow man.
Say I mow your lawn. For doing so, you pay me $20. I go to my grocer and demand, "Give me 2 pounds of steak and a six-pack of beer that my fellow man produced." In effect, the grocer asks, "Williams, you're asking your fellow man to serve you. Did you serve him?" I reply, "Yes." The grocer says, "Prove it."
That's when I pull out the $20 I earned from serving my fellow man. We can think of that $20 as "certificates of performance." They stand as proof that I served my fellow man. It would be no different if I were an orthopedic doctor, with a large clientele, earning $500,000 per year by serving my fellow man. By the way, having mowed my fellow man's lawn or set his fractured fibula, what else do I owe him or anyone else? What's the case for being forced to give anything back? If one wishes to be charitable, that's an entirely different matter.