EVERY AMERICAN supports tax simplification in general, but nobody wants to give up the particular complexity that benefits him. That is the point on which the U.S. Treasury is not struggling with Reps. Barber B. Conable (R-N.Y.) and Joseph L. Fisher (D-Va.) over their tax amendment. They are both congressmen of great ability and devotion to the public interest. In this case they happen to be wrong, but that does not make the point any less interesting.

For some years, Congress has been trying to simplify your tax return. One of the few practical ways to do it is to raise the standard deduction. That cuts the number of taxpayers who have to itemize all their deductions for interest, medical costs, gifts and so forth. But currently there is a rising chorus of complaint from charities and fund-raisers. If people don't itemize their deduction, they don't get any tax advantage by giving to charity. The tax advantage, the charities claim, is the great magnetic lure that keeps people giving.

There is an ideological point here. The individual taxpayer, giving the money where he chooses, supports a far greater diversity of values and purposes than a government can. If the universities, museums and theaters had no source of funds but the government, and highly valuable diversity would vanish from American life. We agree wholeheartedly with the people who argue that a steady flow of charitable giving, to all sorts of institutions, creates necessary balance to the power of the government.

It is the purpose of Reps. Conable and Fisher to encourage charitable giving. So far, so good. To that end they wrote their amendment - which the Ways and Means Committee has added, provisionally, to the Carter tax-cut bill now before it. The amendment would let taxpayers take the standard deduction - and then itemize charitable donations on top of it.

That's what Mr. Conable and Mr. Fisher get into trouble. The amendment would be hugely expensive. Whether it would have any effect is highly questionable. If it should prove wholly ineffective - that is, if people only took deductions on the contributions they already make, with no increase at all - the amendment would cost about $2 billion a year. If it succeeded in evoking greater giving, it would cost more. Since the authors do not favor increasing the budget deficit, would they make up that $2 billion-plus by raising the income-tax rates?

About 70 percent of all taxpayers take the standard deduction. They give, on the average, about $165 a year to charities, of which religious donations are a very large proportion. But religious donations are not responsive, experience suggests, to tax preferences.The gifts to colleges, orchestras, hospitals and so on are highly sensitive to the tax laws. But those gifts come preponderantly from the people who already itemize their deductions and who would not be affected by the amendment. The Ways and Means Committee would be wise to reverse itself and take the amendment out of the bill.

One thing that the amendment would certainly do is to make it much more complicated to fill out a tax return, and to audit it. The purpose of the proposal is to induce 60 million taxpayers into practices requiring records, and proof, of all donations. The Internal Revenue Service would have to set up procedures to check them. But, although the charities are grossly exaggerating any benefit that the amendment is likely to bring them, who wants to say no to a charity? They are such nice people. The history of the Conable-Fisher amendment helps explain the extremely show progress toward the great goal of tax simplification.