Jack Renner, one of the PGA tour's bright young players, has spent much of his first 24 years banging golf balls into the darkness at various and sundry driving ranges around the country.

Hour after hour, swing after swing, one vision has invaded his thoughts: "I picture myself walking off the 18th green and I've won the Open," he said today, relaxing in the Merion Golf Club locker room. "Every player who has ever swung a club has dreamed of winning the U.S. Open, or any of the 'majors' for that matter.

"We're all fans, too. We've grown up watching these things. You have to be affected by that."

Because he is affected, Renner has developed a theory about major tournaments. "If you could bring up a young golfer and never let him see a major, he'd have 10 times more chance of winning one," Renner said. "The way things are, anybody who's in position to win one of them starts thinking" -- Renner began quaking in mock fear -- "'My God, I'm leading the Open! What do I do now?'

"What you have to do is just play like it's any other golf tournament. But inside your head you know better. It isn't just any other golf tournament. It's the Open or the Masters. No way it's the same."

The mystique that has grown up around the four majors -- the Masters, the U.S. Open, the British Open and the PGA -- is not, as one might suspect, media created, although the crowds stumbling around here in the heat preparing for the 81st U.S. Open that starts Thursday might suggest otherwise. This mystique is player created.

"When you're gone, the others who play this game will measure you by what you've done in the majors," said Ray Floyd, who has won two, the 1969 PGA and the 1976 Masters. "That probably isn't right but that's the way it is. Personally, there's no way I'd trade my other 13 tour wins for my two majors if I had to choose. But a lot of guys feel different."

Many feel differently because of Jack Nicklaus, the 41-year-old all-time champion of majors who has won 20. In recent years, Nicklaus repeatedly has said that all he cares about now is winning the majors. Some players resent that attitude, but others understand it.

"He can afford to take that position because he's on top looking down," said George Archer, who won the 1969 Masters. "Personally, I think the easiest tournament to win every year is the Masters because it has the weakest field. The second easiest is the Open because it has the second weakest field.

"But Jack likes to win majors. That's fine, he's entitled. Me, I'm just as glad to win at Greensboro."

But winning at Greensboro does not bring endorsements, appearance contracts or worldwide recognition.

"Personally, I've always tried to treat the majors the same as any other tournament," said Bruce Lietzke, who has won three tournaments this year and is third on the money list. "I would rather be the leading money winner than win a major.

"But I know that if I'm going to be recognized in the future, I've got to win a major. That's what people look at. It's all a matter of perception, but that's the way people perceive things. I wish you could put something in the water and change it though."

Lietzke freely admits that he developed a negative attitude towards the majors in 1977, the first year he played the Masters. He arrived in Augusta, Ga., that year the leading money winner on the tour, having won two tournaments early. He was swinging superbly, dripping with confidence. He walked into the press tent and sat down, having played his first practice round. The first question came at him:

"How do you feel coming in here knowing you can't possibly win, even though you're the leading money winner?"

Lietzke answered politely, but walked out mumbling to himself about the press and its silly perceptions of the Master and the majors. "I started telling some of the other players about it," Lietzke remembered, "and they all said, 'Oh, but they're right. There's no way you can win a major the first time you play in it.'

"That really upset me.I went around with a bad attitude toward the majors for awhile after that. I was never happier than when Fuzzy Zoeller won the Masters in '79, playing in it for the first time."

Before the 1978 U.S. Open, Andy North was one of the rapidly rising players on tour. He had gone from 53rd on the money list in 1975 to 18th in 1977. That year, at the Open, North put it all together and won, making all the dreams of boyhood come true.

He has not been the same player since.

"I started thinking I was going to win every tournament," North said. "I'd won a major, so heck, I could win these other things. I lost my concentration, the edge that had made me a good player. I'm only now starting to get it back."

North finished 14th on the money list in 1978, largely because of his Open victory. Since, he has dropped to 54th, then 69th. This year he is 96th.

"The toughest thing about the majors is keeping yourself under control," said Ben Crenshaw, who has been second in three of them. "You get out there Sunday and you start getting tingly, all excited. You can't think ahead. More than any of the other tournaments, brains are what win in the majors. Lots of guys have the swing to win them, only a few have the brains."

Much of that thinking has to do with the courses the majors are played on. The courses are generally among the toughest in golf; if they're not, tournament officials usually grow the rough long enough to make them that way.

"A mistake hurts you more in a major," said Rex Caldwell, who led the 1979 PGA after three rounds, then finished 20th. "You can't get greedy and go for birdies like on other courses. You know you have to be thinking all the time out there, but the majors do something to your mind."

People also notice when you blow a major. Tom Watson carried an unfair "choker" label for several years after leading the 1974 and 1975 U.S. Opens through three rounds and failing to win either time. It was not until he beat Nicklaus head to head in the 1977 Masters and that year's British Open that his greatness began to be generally recognized.

Even now, with three British Opens and two Masters to his credit, Watson constantly is asked about not having won the U.S. Open or the PGA. And he is asked about Nicklaus' 20 majors.

"Any time you win a golf tournament it's something special, no matter which one it is," he said."Of course, there's something different about the majors because of all the attention paid to them.

"I want to win the Open, absolutely. I've never won it so it is important to me. But it isn't an obsession or anything."

Nicklaus checked into the Cleveland Clinic last Thursday because he has been suffering from diarrhea for several months, he said today.

Lee Trevino, who won here in 1971, also was in a hospital recently. He received cortisone treatments for his back, injured six years ago when he was struck by lightning. "I got to feeling too good and tried to lift some heavy things in my garage," Trevino said.

Arnold Palmer will play in his 28th Open. His last top 10 finish was ninth in 1975.Seve Ballesteros, after last year's flap over his disqualification, is politely refusing interviews this year.