Some opponents of the flat tax will concede all of the above, but argue the flat tax is suspect on Smith's fourth criteria, distributional equity, the fairness test. Moreover, some add, because the millionaire owns more property, he or she receives more protection from the government and should shoulder a greater proportion of the tax burden than families living from paycheck to paycheck. I am not inclined to argue over such things. They seem almost irrelevant in the face of the perception that our present tax system is not fair, that it does not equitably distribute the burden because of loopholes enjoyed by influential fat cats. Even if sincere and thoughtful critics of the flat tax are right on the equity issue, there is no objective, scientific measure for fairness.
Equity depends upon values that transcend objective analysis.
What is fair is what people perceive to be fair, given their knowledge and value systems, and the current federal income tax system is not perceived to be fair. It is considered by some brutal and tyrannical. The simplicity of one single tax rate paid by all regardless of the level or source of their incomes appeals to the simple sense of fairness of ordinary citizens. At least, it seems fairer to many of us than what we have now. There are a couple of other strong reasons to support a flat tax.