Baseball players learn early to believe in absolute distinctions, lines of demarcation as clear as Out and Safe.

One of the game's basic assumptions is that the ultimate testing ground of the sport is a pennant race.

Neither players nor managers argue this. It is a given, century-old a priori proposition.

The World Series is a grand celebration, a showcase and circus. The playoffs are Russian roulette, an inescapably inequitable best-of-five series that overemphasizes top starting pitchers and one-man bullpens.

But in a pennant race excuses and niggling reservations disappear along with smiles. Something in every sport must be of unquestioned value and in baseball it has been decided that the league (now divisional) flag is that sine qua non.

A man can blame a week's bad luck for postseason woes, but copping a plea for losing over a 162-game track is considered the act of a busher, a loser.

Baltimore manager Earl Weaver, his team winning at a furious pace this month, but gaining not one inch on the New York Yankees, blurted out last Saturday, "We've got 93 victories already. That's as many as the (Los Angeles) Dodgers and they clinched their division a week ago.

AL East Race at a Glance

"Where's the wild card in this deal?"

That's the point. Of the major sports, only baseball has no wild card, no back door to the playoffs, no consolation winner.

If three teams of 600 caliber (New York, Boston and Baltimore) happen to find themselves in the same division, then that's just tough. Two go home flat empty. The payoff for second place in thedivision is zero.

"This is throat-cutting time," said Boston's Carl Yastrzemski two weeks ago before the American League East's head-to-head wars began. "I wake up every morning at 5 a.m. and can't get back to sleep. I can't wait for it to be time to go to the park.

"In a pennant race your whole life stops," said Yaz, who is 1967, 1972, 1975 and 1977 has risen to his greatest performances under September tension. "Only one thing matters - the next game."

Baseball is customarily a leisurely game. Mistakes even out. Slumps and streaks are ground down inexorably to lifetime averages. "When I'm hitting 300," says New York's Reggie Jackson, "I know something bad is probably coming. When you've hit 265 for 10 years, you know that's who you are."

But in September averages mean nothing, only the moment counts. Nevertheless, players are still bedeviled by past performance.

"It's hard to forget you're a 235 career hitter," said the O's Mark Belanger two weeks ago. "But for pitchers it may be even harder. I hope Rudy May doesn't start thinking about how he's been a 500 pitcher all his life (actually 105-116). We need him to think like a 20-game winner won."

At that point, May was 17-11 with his first even remote chance for 20 victories. Since, he has lost three straight, playing a major role in the Orioles' slow slippage.

"No luck. I can't get any luck," said the intelligent, genuinely distraught May after 3-1 and 3-2 losses in consecutive starts.

But some hurlers make their luck. Baltimore's Jim Palmer has only one loss in six weeks. That's a stretch run. When the other fellow gives up one run, Palmer gives up none. September demands no less.

No pitcher has complete respect from his mates until he has passed through these fall fires. The Orioles watch rookie Dennis Martinez blow away the Red Sox with nine Ks and Ross Grimsley loses to Toronto and Cleveland. Few will forget. Wonder which 'll be in the rotation next year?

Boston sees partly Reggie Cleveland through his two best games of the year at the Yanks within a week. And the New Yorkers marvel at scrawny Ron Guidry who has one flawless outing after another.

And what of those who call in sick? Will they be forgiven? Mickey Rivers, with memories of bolts thrown at him from the bleachers in June and a Cleveland fast ball in his ribs a week before, remembers a muscle pull and asks out of a crucila game in Fenway. The next night he decides to play and fields a fly ball with his face.

Not a Yankee leaves the dugout to see if Rivers is conscious.

Baltimore's Doug DeCinces does not miss a game despite a jammed and swollen thumb. He has to bunt for hits once or twice a game because the pain is too bad to warrant swinging unless there are men on base. "Earl says he needs my glove in there," says DeCinces, proud to let a year of batting-average building go down the drain to help the team.

The entire atmosphere of pennant race games is unique. The well-heeled fans at Series games, so proper, even if partisan, are absent. The crazies, the be ligerents arrive in September, trying to reach out with their voices to grab the visitors throats. Listen well, the hatred is real.

"Usually, if you look up and smile at the people who are heckling you, you'll win them over," offers Cleveland. Teammate Bill Lee looks at him incredulously. "You smile up into the stands in Yankee Stadium, Reggie," advises Lee, "and you may get a dart between the eyes. In '75 I saw a guy pull one out of his arm."

Perhaps the most inexplicable quality of these last two weeks is the undeniable and almost eerie theatricality of nearly every game.

Many a midseason game has that flaw that Churchill once found in his pudding - it has no theme. But games in the fall seldom suffer that problem.

Teams react more to the flow of the action now," admitted Boston's Carlton Fisk. "Whoever makes the first dramatic play seems to get an enormous edge."

So Fisk dove three rows into the box seats to make a miracle catch of a foul pop in the first inning and that play seemed to hang over the next two ours like a premonition of defeat that he opposing Yankees could not shake.

Players pride themselves on forgetting yesterday's game. It is essential for reaching the majors. The players who lets "oh-fers" (as in 0-for-4 gnaw at him cannot survive.The game will have him for lunch.

But in September, each day's code seems to carry over as the following day's prelude.

The Orioles lost a gut-twister in Cleveland on Friday night, then played like zombies for the first seven innings the next day until a splendid catch by Al Bumbry, preventing a grand slam homer, woke them up.

In their next ups, the Birds hit back-to-back ninth inning homers and won. The carry over countinued the next afternoon as the O's scored three runs before them made three outs.

But the cycle reversed again. One mid game mistake bred others and suddenly Baltimore committed four errors and a passed ball in a single inning to throw throw the game away.

A few players thrive on this roller-coaster of emotion and even make it serve them. If one player epitomizes the pennant race performer, it may be that maligned $3-million candy bar, Reggie Jackson. In June his verbosity and erratic fielding, stood out. His manager wanted to punch his ticket.

But down the stretch Jackson's irrepressible theatricality consistently lifted his team and depressed the opopsition. No player in the American League approaches Jackson as a spot-light hunter. He lusts for attention. Pure, limitless, dizzy-headed adulation is his goal, but, if necessary, even being reviled suits him better than unnoticed mediocrity. And that is the temperature that September so frequently rewards. "Dare to excel," is the credo.

The single most dramatic swing of these last two weeks, as New York has taken command of its division, was Jackson's ninth-inning homer breaking up a scoreless game with Boston.

His two outstanding catches in that game may have been doubly valuable for the wat he made them look even tougher than they were. Strange, but probably true.

Jackson has a way of sending messages. He knows that other teams get their scores by Western Union ticker.

"Jackson homer, first, two on," was the message that awaited both Boston and Baltimore on Sunday befoere their games began.

When the zip has gone out of the last pennant race of a season - the annual six-month argument among old enemies finally settled - the baseball year has usually exhausted its most pungent and subtle pleasures.

The postseason extravaganza pleases the mass of fans, those one-a-year baseball lovers. But to the players and those noncombatants who have gotten closest to the game's skin, testing its rapid pulse every day, these past two weeks were the time to see baseball distilled to its most stunning crystal.