Income received in recompense for human effort.
In my view, shifting from ‘productive contribution’ to mere ‘effort’ still leaves us with no solid basis to justify/critique income.
1. Pure (absentee) owners may argue that, while they exert no current effort here and now, their income compensates for past effort — theirs or their predecessors’.
2. For those who do exert effort, we don’t know whether their effort affects production positively or negatively, and by how much.