Thanksgiving week marked the traditional start of the holiday season, when our thoughts should turn to those less fortunate than ourselves. Most charities aimed at helping others report their heaviest donations during the holidays, and not only because of the urgent reminders besieging us on every side.

As the end of the year approaches, families measure out their remaining disposable income in order to decide how much to contribute during the great year-end giving binge. And one of the things most people take into account is the tax deductibility of their donations.

Maybe in a perfect world nobody would worry about whether gifts to a particular organization were tax-deductible. In the imperfect world in which we live, however, charities fight to preserve their 501(c)(3) status, and with good reason: The deductibility of contributions affects people’s willingness to give.

This proposition is a useful starting point for analyzing the rising mania among politicians on both sides of the aisle to adopt a policy long popular within academic circles -- either eliminating or severely restricting the charitable deduction, at least in the upper-income brackets. For some, this would be part of a larger tax overhaul, reducing rates in return for capping or abolishing various deductions. For others, the idea is simply to tap a pot of gold that seems to be lying there, while leaving most of the current structure of the tax system intact.

By all accounts, the pot is a big one. According to the Obama administration’s budget estimates, the amount by which itemized contributions reduce the personal income tax will total about $53.7 billion in fiscal 2011, and about $315.1 billion over fiscal years 2011-2015. That is a lot of money, and it is a natural place for a cash-strapped federal government to look for revenue. Natural, but mistaken.

The deduction for charitable contributions is one of the oldest in the tax laws, dating back to the War Revenue Act of 1917. Congress understood from the start that it was creating an incentive to give. Senator Henry Hollis, a New Hampshire Democrat, explained during the debate that people contribute to charities “out of their surplus” -- after paying other bills - -and, therefore, in difficult times would contribute less. Without the deduction, he said, charitable donations “will be the first place where wealthy men will be tempted to economize.”

The theory behind the deduction, as Rep. Carl Curtis, a Nebraska Republican, noted during World War II, is that charitable giving is “exempt from taxation” because it represents “an expenditure for the public good.”

Curtis had it right. Charitable giving represents spending for precisely the same purpose that the government spends: to promote the general welfare. The difference is that the individual who gives to charity might measure the needs of the community by different calipers than centralized policy makers, and will therefore contribute to a different set of causes.

These millions of individual decisions lead to a diversity in spending that would be impossible if we adopted the theory that the only money spent for the public good is the money spent by the state.

Democrats and Republicans alike have come to refer to the charitable deduction’s “cost” to the government. This language, however, only makes sense if one concedes that government has first call on every dollar earned in America. It isn’t obvious why this starting point is the correct one. It is just as easy to begin with the proposition that the earner owns the money, in which case it is the removal of the deduction -- or the tax itself -- that is a cost.