It's the legislation, not its implementation, that makes solitary confinement a harmful prison practice that must be banned, a rights group told court on Friday, capping arguments in a suit that tests the legality of the controversial procedure.

The matter of whether the Correctional Service of Canada's use of administrative segregation passes constitutional muster now rests with Ontario Superior Court Chief Justice Frank Marrocco.

Over the previous three days of the hearing, he heard lawyers for the Attorney-General of Canada argue that any harm done by solitary confinement was a matter of "maladministration" rather than legislative gaps. Crown lawyer Peter Southey told the court that the Correctional Service learned of the dangers of solitary relatively recently and worked dutifully to address them with a flurry of changes to internal policies and directives.

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Read more: Solitary confinement: How four people's stories have changed hearts, minds and laws on the issue

On Friday, Canadian Civil Liberties Association (CCLA) lawyer Jonathan Lisus summarily rejected the Crown's position.

"It's simple: This legislation and this practice is an artifact, a cruel artifact, from another time when the consequences of this practice weren't understood," he said. "And now that it's become a bureaucratic reality, there's no principled, rigorous, bureaucratic, medical, scientific or constitutional justification for 23 hours a day in a cell. There needs to be a fresh look."

The focus of the case is a handful of provisions in the Correctional and Conditional Release Act that authorize the use of prisoner isolation.

Mr. Lisus told the court that the act was written in 1992, before the hazards of solitary confinement were fully known.

The association presented a large body of evidence suggesting that solitary confinement has since been linked to an array of negative health impacts, including various forms of mental illness and suicidal behaviour.

The Crown countered that the legislation, though dated, already accounts for any health effects. The act states that all inmates are entitled to the same level of health care as all other citizens and that consideration must be given to their "state of health" prior to being placed in solitary.

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Mr. Southey contended that recent policy changes have only reinforced that health-care requirement. Among those changes, some of which were adopted as recently as August, are prohibitions on segregation placements for several vulnerable inmate populations, including pregnant women, the physically impaired and "inmates with a serious mental illness with significant impairment."

Mr. Lisus seized on that latter rule as evidence that new rules "compound" problems in the legislation. "I'm sorry, Chief Justice, but this sets a cruelly high bar," he said. "We are supposed to prevent these disorders from blossoming, not simply look for them and take them from segregation when they present. … This isn't care, this is crisis management."

The proceeding ventured briefly into debates over emerging brain science. The CCLA said the still-growing brains of young inmates make them particularly susceptible to the adverse effects of isolation. The Crown, meanwhile, showed evidence explaining that while the brain continues to grow for the first several decades of life, the essential structure is fixed by the late teens, reducing sensitivity to mental stressors.

The CCLA filed suit in January, 2015, alleging Charter violations. It wants the administrative segregation provisions in the act struck and rewritten to include 15-day limits on segregation placements, independent oversight of segregation decisions and prohibitions on isolating inmates who are mentally ill, requiring protection or aged 18 to 21.

The trial focused largely on the current solitary-confinement regime rather than a bill of proposed changes to segregation introduced by the federal government in June. The CCLA has said neither the current system nor the new bill would prevent inmates from being isolated indefinitely.

It is one of several cases currently winding through the courts that challenge the use of solitary confinement by both federal and provincial corrections systems. A similar lawsuit in Vancouver brought by the BC Civil Liberties Association and the John Howard Society of Canada is also awaiting a judge's decision.

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The Globe and Mail has reported extensively on the prevalence and effects of solitary confinement, beginning with a 2014 investigation into the suicide of Edward Snowshoe after 162 consecutive days in segregation.