Whether they are organized along benefit or ability to pay lines, most modern tax systems also attempt to incorporate modern views about fairness or equity. One important principle is that of horizontal equity, which states that those who are essentially equal should be taxed equally.
The notion of equal treatment of equals has deep roots in Western political philosophy. If you and I were alike in every way except the color of our eyes, all principles of taxation would hold that we should pay equal taxes. In the case of benefit taxation, if we receive exactly the same services from the highways or parks, the principle of horizontal equity states that we should therefore pay equal taxes. Or if a tax system follows the ability to pay approach, horizontal equity dictates that people who have equal incomes should pay the same taxes.
A more controversial principle is vertical equity, which concerns the tax treatment of people with different levels of income. Abstract philosophical principles provide little guidance in resolving the issues of fairness here. Be warned that general and abstract principles cannot determine the tax structure for a notion.

Does the weighted-pupil approach add objectivity and equity to the school finance system by providing equal amounts of money per pupil and paying more per pupil for higher-than-average-cost programs?