Psychiatrist David Bear came on strong at the trial of John W. Hinckley Jr. It seemed that Dr. Bear, a 39-year-old Harvard professor with bushy black hair and an air of certitude, wished to convince the jury that psychiatry is an exact science, with iron rules and inescapable conclusions.

He was the antithesis of the noncommital, "What-do-you-think?" clich,e of the shrink. He was a sovereign power in the courtroom, pitting himself against the authority of the judge, Barrington Parker, expressing his views in combative, aggressive terms calculated to discourage argument from the prosecution.

"That is absolutely false," he said emphatically when prosecutor Roger M. Adelman suggested that young Hinckley, who considered shooting actress Jodie Foster and then himself, or the president, chose the option of shooting Ronald Reagan as he emerged from the Washington Hilton at 2:30 p.m. on March 30, 198l.

"Option," said Dr. Bear didactically, "means a choice available to a man with a clear, rational mind. He was a man on a roller coaster, with jumping thoughts."

Dr. Bear fought Adelman through every sentence of a government psychiatric report which showed Hinckley considering not shooting the president because it was raining.

"That wasn't driven, that wasn't compelled," Adelman suggested.

"This doesn't shake my confidence one bit," replied the arbitrary doctor.

Patients are the worst possible source for reliable information about themselves, he said. Casual observers are a close second, he indicated, dismissing the opinion of a former District of Columbia medical examiner, who said of Hinckley hours after the shooting, "I had no feeling this was a classical nut."

Hinckley is pleading not guilty by reason of insanity. Dr. Bear diagnoses his illness as schizophrenia.

The defendant sat at the table, the center of a ring of marshals, some facing out at the press and the spectators, who included his mother and father, two well-dressed pillars of suburban respectability and achievement. John Hinckley Jr. has a round, almost childish face and dead eyes.

He seemed rather bored by the diagnosis being offered so vivaciously in his defense by Dr. Bear. He knows something about the limits of psychiatry, having been treated for five months in Denver by a local practitioner, who had not found him "a cause for concern" and had discerned no mental illness in him.

There may be a certain rough justice in the fact that Hinckley's only hope lies in psychiatry, since one practitioner had so conspicuously, according to the defense, failed him when most needed.

Dr. John J. Hopper of Denver made little or nothing of the lavish instances of delusions and obsessions which Hinckley had provided: his passion for Jodie Foster, his hopes of becoming a rock star or an author.

Even the defendant's achieving, conforming brother and sister had suggested that he needed hospitalization to correct his demented behavior.

But Dr. Hopper had given what Dr. Bear calls "totally inappropriate, in fact harmful treatment."

He had prescribed Valium and rejection.

According to Bear's testimony, Hinckley took extra doses of the Valium three hours before he gunned down Reagan, a Secret Service agent, a policeman and the president's press secretary, James Brady.

As for the expulsion from his parents' home, it had been accomplished with heartbreak. It was recounted by JoAnn Hinckley, his handsome, black-haired mother, who broke down on the stand when remembering how she told her son he could not come home again.

"I am sure that was the greatest mistake in my life," the father, a distinguished-looking business executive, had told the court. "I am the cause of John's tragedy."

Outside the courtroom, the right and wrong of it were debated in the line of spectators, which on Thursday morning included a dozen members of the senior class at Chantilly High School, looking for credits in a government course.

"It was kind of cruel," said one curly- headed boy with braces on his teeth, "throwing him out of the house like that. I know they were advised to do it, but still . . ."

"It was a parental nightmare, that's what it was," said another student.

The young people, who were nearer in age to Hinckley than the members of the jury, seemed to feel that Hinckley should be convicted. They thought he was "crazy" but must have known what he was doing when he fired at the president.

"No matter who he shot," said one of them, "you can't have people like that running around in society."

Dr. Bear was not asked, nor did he volunteer, how John Hinckley Jr., who was once a cheerful child and a basketball star, who was brought up to be happy under the most promising circumstances, grew up to be a presidential assassin. Enlightenment on that point might give his trial some redeeming social value.