As TAX-DEDUCTIBLE outrages go, the business lunch is somewhere down around the middle of the list. In outrage quotient, the deductible lunch certainly ranks lower than the lawyers' professional seminars in centers of learning like Acapulco and Monte Carlo. It is lower than the deductible yacht, on which the owner entertains clients (and himself). A lunch, in even the most elitist of French restaurants, is somewhat less offensive as a tax deduction than the country-club dues of the salesman who alleges that, between swims, he prowls there for prospects. In comparison, the lunch is only a semi-outrage.

The Carter administration, with majestic logic, proposes to make it a semi-deduction. If you take a customer to lunch, the customer's lunch is business, but yours is pleasure and therefore should be taxable. After all, you'd otherwise be eating in the cafeteria. But, the president sternly adds, the time has come to abolish all the other deductions for business entertainment - not by half, but altogether. The tax bill now before Congress would do just that. It deserves to be enacted. If a corporation wants to spend money on its customers and its executives, that's its own business - as long as it spends its own money. But a tax deduction is a subsidy. The statutory rate on corporate income is now 48 percent, which means that the U.S. Treasury, through the taxes not collected, bears 48 percent of the bill for those deductions.

The principle here is a basic one: Business deductions ought not cover perquisities that improve a tax-prayer's personal standard of living and comfort. One trouble with these perquisiter is that they are available only to certain classes of people. They are routinely available to lawyers and doctors, corporation managers and salesman. They are not available to secretaries, plumbers or bus drivers. That distinction is invidious and unwholesome.

Let us try to anticipate the Letters to the Editor that will point out, angrily, that newspaper advertising is a deductible business expense. If a deduction is permissible for advertising, why not for business entertainment - which is, arguably, a form of public relations? The answer is that nobody regards reading ads as a particularly enticing form of recreation, in contrast to football games and plays attended on expense accounts. In any case, ads are open to everybody, but expense accounts are not.

The Treasury cites the case of a man who claimed a deduction of $9,665 for 338 business lunches in one year - some of them, evidently, on Sundays. There was the surgeon who deducted $14,000 a year for his yacht; he used it to entertain other doctors, who might refer patients to him. (That's another reason to get a second opinion.) A life-insurance salesman deducted his tennis-club dues, on grounds that the game gave him a chance to size up his customers' health. Corporations' hunting and fishing lodges float on deductions of hundreds of thousands of dollars a year. Corporate lodges are fine, if you like that sort of thing. But don't ask the government to pay for them.

The tax system works much better in the United States than in most countries. The reason is that a large majority of Americans accept it as a moral duty to pay. They do it because, despite all the sustem's shortcomings, they accept it as reasonably fair. That presumption of fairness is an asset of immense value, to be protected vigilantly. It is in danger when people see a few of their fellow citizens living extremely pleasantly through the ingenious manipulation of business deductions for entertainment and foreign travel. That is an inequity, and Mr. Carter is calling on Congress to correct it.