It would be reasonable to ask why the government should subsidize charitable contributions at all. But for now, let’s discuss this simpler and more politically relevant question: If we are going to continue subsidizing these donations, what is the best way to do it?

First, I should clarify a simplification I’ve made. In the current system, strictly speaking, your eligibility to deduct a charitable contribution doesn’t depend on whether you have a big mortgage. But it might as well. You can deduct charitable contributions only if you itemize rather than take the standard deduction, and the most common way a household collects enough deductions to make itemizing worthwhile is to have a big mortgage. (Living in a high-tax city like New York can also help a taxpayer cross that threshold, because state and local taxes are deductible, at least for now.)

But I challenge anyone to justify a system in which we essentially subsidize contributions made by people with big mortgages. For one thing, this set-up magnifies the already large distortion created by the mortgage interest subsidy, since having a mortgage qualifies taxpayers for other subsidies as well.

It is equally hard to justify subsidizing the gifts of rich people more than those of poor people. We do so because our tax system treats donations and mortgage interest as “deductions” from income. That is, we subtract these from income and pay taxes on the rest. This is logical when it comes to business expenses. Someone who owns a business should pay taxes only on profits, not on revenue. But that logic does not follow for charitable deductions, which are not a cost of doing business.

What would be better? I suggest three principles to help guide the debate:

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The tax subsidy rate should be the same for everyone. This means that rather than being a deduction from income, the subsidy should take the form of a tax credit, so that if you contribute $1,000 and the subsidy rate is 15 percent, your taxes would be reduced by $150. (Ideally this credit should be “refundable,” so it is payable even if your tax bill is zero or negative.)

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In the interest of tax simplification, the tax credit should apply only to donations above a certain minimum. The minimum could be, say, 2 percent of adjusted gross income. That way, only large contributors need to bother keeping records.