once you get into the complexities that apply to investors and rich people, income is a slippery concept. you have a hot dog stand and pay yourself 10k, is it salary, dividend, capital gain, return of capital? maybe you didn't even make money after depreciation, inflation.
"Income, like sausages... are esteemed most by those who know least about what goes into them." - paraphrasing Alvin Toffler
That's why most economists prefer VAT, simple tax based on the value of work when the work is monetized at any stage of production. (you still need to deal with progressivity, income from capital gains / interest / dividends / wealth creation, the cash flows that cause those to arise are subject to VAT if not the animal spirits but that's another matter)