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Defence counsel Peter Rosenthal knew the Crown's forensic expert had stumbled into his net when she began blithely spouting mathematical terms from the witness box.

According to the prosecution theory, Dr. Rosenthal's client had robbed a woman at knifepoint. Using graphs depicting the rate at which dye was absorbed into microscopic clothing fibres located on the knife and inside the defendant's pocket, the expert claimed that the knife must have been his.

Dr. Rosenthal pressed the witness for a detailed explanation, prompting the expert to say it was all too complicated for laymen to understand.

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"Get as complicated as you want," replied Dr. Rosenthal, 65. "I'm a mathematician."

Not just any mathematician, in fact, but a well-respected, 40-year veteran of the University of Toronto's mathematics department.

"She got incredibly flustered and started sputtering," Dr. Rosenthal said in an interview. "She really didn't have any technical explanation. I think it was what led to my client being acquitted."

The incident was a direct result of a highly unusual career path that saw Dr. Rosenthal, midway through his academic career, pursue a law degree at U of T's renowned law school. Since then, his gravelly New York City accent has become as familiar in the courtroom as it is on the campus.

What makes him unique, however, is his choice of clients and his fee structure. A lifelong Marxist, Dr. Rosenthal offers the vast majority of his legal services free of charge to a spectrum of leftist causes. He has devoted thousands of hours to defending demonstrators arrested for trespassing, injured workers denied compensation, victims of police brutality, and squeegee kids who challenged an Ontario law that prohibits them from approaching cars.

He is also active in the civil courts, suing police, civic officials -- even a university -- on behalf of poverty activists and protesters.

Dr. Rosenthal has stalked bigger game. In 2003, representing the Communist Party of Canada, he persuaded the Supreme Court of Canada to strike down a law preventing small political parties from enjoying the same tax deductions as large parties. Last fall, he won a parallel victory when an Ontario judge said that small political parties are entitled to the same allotment of $1.75 a vote that big parties get.

"If all the lawyers in Toronto sat in a big hall and someone asked for those who stood in solidarity with the poor, a sprinkling would stand up," said John Clarke, the Ontario Coalition Against Poverty activist, who has been arrested in several demonstrations. Dr. Rosenthal, he said, is in a small minority of those who are willing to act on their beliefs. "That they will work selflessly, for little or no financial return, enables us to defend ourselves without being wiped out by debt."

Born in New York on June 1, 1941, Dr. Rosenthal was raised with strong Marxist principles. His mother, in particular, was very active in the civil rights and anti-war movements. On leaving the University of Michigan with a mathematics degree in 1967, Dr. Rosenthal landed a faculty position at U of T and became politically active. "I regard myself as a Marxist, but not one affiliated with any particular parties," he said. "I have a very strong hatred of racism and the grotesque economic inequalities such as exist in the world. It is very deeply embedded in my bones."

While hollering through a bullhorn at the U.S. embassy during a Toronto demonstration in 1969, Dr. Rosenthal was hauled to the ground by police and charged. At his trial, he interrupted his lawyer so often with advice and suggestions that the trial judge eventually asked which one of them was going to try the case. Dr. Rosenthal fired his lawyer and took over.

He was ultimately acquitted of obstructing police but convicted of causing a disturbance. He appealed the conviction and won. Dr. Rosenthal was hooked on the law.

Studying law at night and acting as a sort of paralegal, he took on more and more cases, representing clients in human rights, coroner's inquests and workers' compensation cases. Dr. Rosenthal represented the Black Action Defence League in its battles against Toronto police, the Innu in their fight against low-flying military planes, and numerous alleged trespassers who had picketed a Litton Systems plant in Rexdale that designed guidance systems for Cruise missiles.

Dr. Rosenthal's law degree enabled him to defend more serious charges, but his clients remained social activists, outcasts and the oppressed. "I'm all work and no play, but I don't do either of my careers for money," he said. "I really like doing mathematics and I really like the law. I'm very lucky."

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While lawyers who share his convictions often blunt their courtroom effectiveness by indulging in provocative political statements or berating judges and prosecutors, Dr. Rosenthal has steadfastly rejected the temptation. "I think most people see that I'm up front about my beliefs, and I pursue my causes honestly," he said.

"He is an exceptionally imaginative and tenacious lawyer -- and those are wonderful qualities when they come together," said Clayton Ruby, a leading criminal lawyer for the past 40 years. He said that, in the profession, Mr. Rosenthal is admired not so much for legal brilliance as for dogged determination. "Those who make the greatest contribution are rarely the most brilliant lawyers; it's courage that makes the greatest lawyers."

Mr. Clarke said that, as a client, he marvelled constantly at Dr. Rosenthal's ability "to develop arguments of enormous detail and complexity, and then to apply them in the cut and thrust of a courtroom setting that is hostile in the extreme. He behaves like an intellectually brilliant bulldog."

Mr. Clarke also recalled being struck that Dr. Rosenthal was such "a thoroughly decent human being." During one trial that followed a violent demonstration outside the Ontario legislature, the Crown played a speech in which one of the demonstrators expressed outrage at the suffering of homeless people.

"I happened to look over at Peter and his eyes were full of tears," Mr. Clarke said. "His abilities and degree of conviction are enormous, but his compassion is what makes them powerful."

Yet his legal triumphs have not detracted from his initial calling, said Lee Lorch, a mathematics professor at York University.

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"He makes valuable contributions to Canada through his excellent mathematical research, his training of students and, perhaps most notably, by his passionate volunteer work for social justice," Prof. Lorch said. "His clear insights, personal courage, unbounded energy and determination combine to help us understand the need to be ever-vigilant for justice -- and the importance of constant struggle to achieve it."

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