Some things we can believe about Pat Fischer. Put him in a three-piece suit and yes, he can pass for a banker. Give him jeans and boots, he's a horseman, both a breeder and a racer. But there is no way to imagine Fischer playing professional football. A cornerback? Forget it. Raquel Welch isn't built to be a typist, and Pat Fischer isn't built to hurt people.

The first thing a tourist notices about the Washington Redskins' training camp is that these guys are big. These guys are buildings. When George Allen wants to talk to Terry Hermeling, he takes the elevator to the 11th floor.Backpackers go on overnight hikes around Dave Butz. In professional football, a little is one who doesn't worry about light aircraft running into his ears.

When football coaches make out a list for Santa Claus, they ask for a sock full of defensive backs who are 6-foot-2 and 200 pounds with wonderful speed. Fischer is 5-9, 170. The coaches want those defensive backs young, because old age robs them of quickness and strength. At 37, Fischer is the world's oldest terrifying tackler, ready to start his 17th season in the National Football League, still full of the intelligence, enthusiasm and skill that has made him a phenomenon.

The remarkable thing is that a man with Fischer's body - in shorts at this camp, he resembles nothing if not a jogger who got lost - not only has succeeded at a position where coaches long for great athletes, but he's also done it forever. It's all in the tackling, Fischer said. "It's not getting hurt.

"Most careers are ended prematurely by injury. Most players don't get old and lose their speed. They get hurt and lose their speed. Mentally, they give up. The frustrations of accepting challenges week after week, even day after day - the challenges of young players coming up, the coaches pushing you - you always have those challenges to deal with.

"And then, say, you have an injury. To suffer extraordinary pain; to wonder how the injury is going to affect you later; Will I be able to play golf? Go hiking? Play tennis? And then to have daily tapping and treatment; it mentally wears you out. You say, 'I will find another job.'"

Maybe so. Maybe that's the way it works with a lot of players. Not with Fischer, who has, in fact, been hurt badly.

In 1971, he had a bad back. "It burned down my leg. I couldn't walk a block." A disc was removed that winter and Fischer didn't miss a game. In the winter of 1975, Fischer could not run. Ligaments in his right ankle were ripped in practice. Then, in training camp, his left knee was injured. Although he didn't take part in any practice work, Fischer again did not miss a game.

So he's old, and he's little, and his speed is less than wonderful ("adequate," he says slyly), and he's had his share of injuries. Then what is he doing as a first-string back on a defensive unit that last season set a team record against passing, allowing the enemy to complete only 41 per cent of its throws?

Witnesses to his magic act say that Fischer, at work, somehow becomes as big and fast as necessary. If Isaac Curtis leaps for a pass, Fischer leaps just high enough to tip it away. Those witnesses also say Fischer delivers punishment in extraordinary ratio to his bulk, mainly because he is wise enough to strike when the big guy is least prepared. And if Curtis can't beat Fischer, it's mainly because Fischer knows more about where Curtis is going than Curtis does.

"Knowledge allows me to play," Fischer said. "A knowledge of running dynamics. It's recognition of the run against the pass. An awareness of where you are getting help, where you linebackers are, where the safety is. If you make a mistake, where should you make it? You have to think."

"He survives because he knows everything that's going on on a football field, and he's so tough he'll take on anybody," said another old man, Billy Kilmer.

It is Fischer's mind and his heart that make him special.

Fischer is a banker, working in Dumfries, Va., and he is a horseman, involved in the Virginia Stallion Station breeding farm, in addition to owning a dozen thoroughbred racers. So how much longer will he stay in the painful game of football?

"I probably should have quit 10 years ago," Fischer said with a smile.

Why go on?

"There are many personal rewards," he said, and then he began talking about tackling, how it was important to take on the ball carrier while he was running laterally, how you should hit him with your shoulder, forearm and full weight of your body.

Fischer was a happy man, lost in his art, and, smiling, he said he learned to tackle playing backyard football with his five mean, strong brothers in St. Edward, Neb., pop. 900.

Small wonder he survives today.