FOR PRECISELY the reasons that Attorney General Griffin Bell offered, the appointment of a special counsel to deal with the Carter family business case is a good idea. The special counsel will be Paul J. Curran, a lawyer who served as U.S. Attorney for the southern district of New York from 1973 to 1975. That is to say: He was appointed by President Nixon. He will now become the outsider who undertakes an examination of the financial relations among the National Bank of Georgia, the Carter family peanut business and the two brothers who are partners in it -- one of whom is the president.

The attorney general said he decided to appoint Mr. Curran in order to reassure the public that the federal inquiry will be carried out "fairly and impartially without even the possibility of deference to high office." With that, Mr. Bell conceded that he is too much an old friend of the president to be able to carry on the investigation within his department despite his efforts to separate himself from it.

You will observe that it is a special counsel that has been established, and not a special prosecutor. The difference is that Mr. Curran will not have independent authority to prosecute; he can bring criminal charges only with the approval of the Justice Department. Is that a crucial limitation? We think not. It is the power to investigate that counts here. Mr. Curran is not inexperienced in this line of work, and he has a reputation for an independent will. If he were to decide that a crime had been committed, the department would hardly find itself in a position to stifle or ignore him.

There are two possible lines of logic that might have led the attorney general to this step. One, of course, is the suspicion, on the basis of his own knowledge of the case, of serious wrongdoing. The other -- and at this point it is certainly at least as likely -- is that the administration is weary of the constant damaging dribble of disclosures about the Carters' peanut business, and it has decided that laying out the whole record will be less costly, whatever the embarrassment, than leaving the explanations to Billy Carter.

Since Bert Lance was running the National Bank of Georgia during part of the period in question, this investigation will necessarily touch the separate inquiry of Mr. Lance's affairs by the grand jury in Atlanta. But that will be incidental to Mr. Curran's responsibility. His job, to put it bluntly, is to see whether money from these sources went illegally, or even improperly, into Mr. Carter's campaign for the presidency.

Mr. Curran will have to wade through the chaotic finances of brother Billy. It looks as though he will find questionable decisions to have been made regarding the peanut warehouse. There is no persuasive evidence at this point of wrongdoing that visibly touches the president. But suspicions and allegations of this nature need to be settled, in public, by a lawyer who is neither friend nor ally. The appointment of Mr. Curran makes this necessary procedure possible.