[_delirium]

> Income tax is really a tax on profit, not total income.

If you run a business, that's largely true, as your sole-proprietor income tax is calculated very similarly to how business profits is calculated. But individuals who earn their income via employment (which is most of them) more often pay a tax on their income rather than profits. Some expenses are deductible, but many of the most common ones are not. For example, commute expenses are typically not deductible [1], even though they are probably the most frequent cost incurred solely as part of earning income. Work clothes are also often not deductible: they are only deductible if they are both formally required by the company, and of a kind that is dissimilar from non-work clothing. So e.g. buying a suit for interviews and/or meetings is not deductible, even if you only bought it for and only wear it for income-earning purposes (I have personally never worn my suit in a non-work context). It's also difficult to deduct the cost of a computer, even if your field is computing and you use it mainly for work and skills development (though it's possible in some cases, if the employer formally requires you to have one at home and doesn't provide it).

[1] "You cannot deduct commuting expenses (the cost of transportation between your home and your main or regular place of work)." http://www.irs.gov/publications/p17/ch28.html