IN CONGRESS -- Where else?--there's talk of surgically removing Social Security from the federal budget. It would be cosmetic surgery, like a nose bob, intended to improve the national state of mind by making spending totals and the deficit look smaller. At his press conference last week, President Reagan was asked about the proposition. "I think it's an interesting idea and to be looked at," Mr. Reagan cautiously said.

He was right to be noncommittal. The idea has been looked at, repeatedly over many years, and it's a thoroughly bad one. The basic purpose of a budget is to provide information and control. The more fragmented a budget becomes, the less information it conveys to people making decisions, and the less control they have over spending.

Social Security, after all, is hardly the only trust fund in the budget. If it is dropped out, why not the highway trust fund, which also has its own earmarked source of revenue? Or the civil service retirement fund? Or the land and water resources fund? All together, the trust funds currently amount to $260 billion a year, which is more than a third of total federal spending.

Over the past decade, there has been a good deal of backsliding from the principle of a unified budget. Last summer, for example, Congress very much wanted to keep building up the strategic petroleum reserve--but it didn't want to add that cost, nearly $3 billion, to the deficit. The solution was to put it "off-budget." That simply means spending the money without counting it in the budget. Off-budget spending, for oil and other things, now runs about $20 billion a year. To get an accurate figure on actual federal spending and the deficit, you need to add that $20 billion to the conventional budget numbers that everyone uses.

Historically, programs have been moved off the budget to shield them from the usual scrutiny and weighing of priorities. It is a device for relaxing the rigor of the normal budget process--a peculiar thing for Mr. Reagan, of all people, to encourage.