We have this word "business" which has some kind of vague meaning, so we attach a bunch of rules to the abstraction...

It isn't quantum mechanics. The rules which the OP professes not to understand boil down to this:

1) Everyone owes income taxes and Social Security/unemployment ("FICA") taxes on all of their income. To restate this in a fun but cartoony way: everyone is a "business". [1] No philosophical debate necessary.

1a) Except: If you get a W2 form along with some of your income, that income already had the FICA taxes paid on it (and, if the estimate is accurate, most of the income taxes, too). This feature is designed (among other things) to make the accounting so easy for most people that, if they are minimally numerate and are willing to actually read the IRS instructions, they can pay taxes with a one-page form without hiring an accountant.

1b) If you can't understand what I have said so far, hire an accountant.

2) If you earn more than a few hundred bucks without getting a W2 form for it, you owe FICA taxes. If you can't or won't figure out what Schedule C is, hire an accountant. [2]

3) Business expenses are deductible. If your non-W2 money is going right back out the door to pay business expenses, you may owe nothing. Keep track of those expenses (throw the receipts in a box, buy everything on a special credit card, hire a part-time bookkeeper, or -- god help us -- learn how to keep an account book). Then declare them; if you can't figure out the forms, hire an accountant.

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Still too abstract for you? Here's rule zero:

0) Hire an accountant. Do what the accountant tells you.

One hour of accountant time, at any point over the last few years, would have been enough to save the OP.

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Now we are in a position to discuss your other point: Why do people often get so harsh when folks propose to "simplify" the tax code?

Let's not ignore the elephant in the room: Mostly, it's because so few of these proposals are in good faith. Most of the tax-related writing that crosses one's field of vision is the direct or indirect result of politically-motivated lobbying efforts by individuals who are trying to lower their own taxes relative to others. We try to block these for the same reason that we block spam.

But another reason is that many people want to pretend that accounting isn't important. They ask: Why does there need to be so much accounting? Why can't life be simple? These are very likely the same people who don't understand the need for so much computer programming. Shouldn't computers just be simpler? Can't we just write it all in Visual Basic? Can't it all be done by kids on minimum wage? Can't we just hand it all over to Microsoft and let them handle it? Why can't we just outsource all that stuff to some foreign country? Why can't computers program themselves? Shouldn't it all just "work"? Weren't things so much better back before we had all these terribly complicated things like the personal computer?

It gets tiresome. Society is complicated. Deal with that. Learn basic accounting, or hire someone who did.

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[1] Note: I am not an accountant and you should hire a real one. You can detect the real ones by the moan of despair that they just made when they read my last sentence.

[2] I can, but I won't. Accountants are not expensive relative to my time. In fact, they have an astonishing tendency to pay for themselves.