Well, that's correct, but I think in general when people here are talking about income, they're talking about wages and salaries, not investment income. So that is what I am referring to by income unless I state specifically investment income.
The constitution expressly provides authorization to tax income from investments. By inferrence, if a tax on earned income isn't specifically authorized, it is forbidden.
Wages are not the same as income. The so-called income tax has been expanded to cover wages when it was originally aimed at income (i.e., profit from rents, dividends, etc.). The federal consititution specifically forbits capitation which is a tax per person. Even the 19th amendment doesn't specifically allow the feds to tax private wages. But that's another war.