The bottom line is that the law presumes that all income is taxable, unless you prove otherwise. “Tax deductions are matters of legislative grace.”
https://ustaxcourtblog.wordpress.com/2015/01/13/tax-deductions-a-matter-of-legislative-grace/
I don’t know if I can think of any other area except for business use of a personal vehicle where the IRS has created black letter rules to help taxpayers allocate a shared personal and business expense.
