Two years ago, when Sandra Day O'Connor was nominated as the first sister to join The Brethren, President Reagan called her a "person for all seasons." The political commentators, on the other hand, called her "a person for all reasons." She was a two-fer: a conservative and a woman.

Now Justice Sandra Day O'Connor has completed her second term at the court with a remarkable finish: she walked down the middle of the road with one foot on each sidewalk. In the court's closing week, O'Connor cast the swing votes in the Norris pension case.

First she agreed with one quartet of justices that pension plans can't pay smaller monthly benefits to women than to men. Then she agreed with the other quartet of justices that this decision should not be retroactive, that equality would start from today.

As Judith Lichtman of the Women's Legal Defense Fund reads it, "She gave us half a loaf." And this is, in many ways, a decent summary of the First Woman's first two years on the bench.

O'Connor has sliced the legal bread on her table in an intriguing way. In most cases, O'Connor voted with conservative Justice William Rehnquist. Indeed their nickname, "the Arizona Twins" could be changed to "the Arizona Siamese Twins."

She voted with conservatives on the death penalty issues and on many civil- rights issues. She helped narrow the standard for class-action suits and agreed that a plaintiff had to prove an employer's "intent" to discriminate. Finally, in the long-awaited abortion case, she wrote the minority opinion that put her squarely in the anti-abortion camp.

But on issues that were narrowly defined as sex discrimination, O'Connor broke ranks. Her very first Supreme Court opinion, in the Hogan case, ruled that the nursing school of a public university could not be closed to men.

Next she ruled that employees as well as students are protected from discrimination in most of the nation's colleges and universities. This year she agreed with liberals on the court that employers had to offer the same pregnancy benefits to spouses of male employees as to female employees.

As Georgetown Law Professor Wendy Williams puts it, "It's remarkable how compartmentalized she is. In all areas she takes the view that states should prevail, except in one area: gender discrimination."

There are a host of theories about why O'Connor deserts the conservatives solely on gender issues. Most of them center around her own personal experiences. In a decidedly sheltered life, the only barriers O'Connor has faced are those marked "No Admittance to Women."

One classic tale has been told and retold about O'Connor. Having been graduated from Stanford Law School third in her class, she was refused a position in every major law firm in Southern California except one. It offered her a job as a secretary.

It's also clear that her ambitions in conservative Arizona were limited because of her sex. Since moving to Washington, O'Connor, an avid golfer, has let it be known that she would like to join Burning Tree Club, a private club to which other Supreme Court justices belong. The club has not yet admitted a single woman. (You let one female Supreme Court justice in and you'll have to let them all in.)

Beverly Cook, a professor of political science at the University of Wisconsin-- Milwaukee who has been studying women judges for the past six years, analyzes O'Connor this way: "It seems to me that she understands quite fully those things which make it difficult for women of her class and position--higher education, insurance. But pensions aren't very helpful to housewives or the poor."

In general, women's-rights advocates agree that O'Connor is "about as good as you could have gotten out of this administration," but there is disagreement on how much she's likely to change.

Cook sees her as "becoming increasingly isolated from other women. She needs another push, but it's unlikely she'll get it now that she's on the court." On the other hand, a highly respected woman judge has said, "This is a warm, generous person. Give her a couple of years."

Time is one thing you get in Justice O'Connor's job. Enough time, it's hoped, to find the other half of the loaf.