In 2010, Kalief Browder was, by all accounts, a healthy, ordinary 16-year-old black teenager living in New York City. On June 6, he took his own life. On Thursday, 12 days later, a Supreme Court justice took notice.

Browder spent three years in Rikers Island without trial for allegedly stealing a backpack. For two of those years, he was kept in “administrative segregation,” more commonly known as solitary confinement. He was subsequently released without ever being charged or tried for the alleged crime. Browder received national attention after The New Yorker reported on his story in October 2014. With the help of an anonymous benefactor, he enrolled in community college and tried to rebuild, but struggled with psychiatric issues he attributed to his isolation. He was 22 years old when he died. Politicians ranging from Mayor Bill de Blasio to Senator Rand Paul cited his case in the broader national discussion on criminal-justice reform.

Now, so has Justice Anthony Kennedy. In a powerful five-page concurrence in Davis v. Ayala, Kennedy criticized the widespread use of solitary confinement in American prisons, which he said affected at least 25,000 inmates in the United States. Among them was Browder, whom Kennedy directly invoked. His evidence ranged from the 1890 case In re Medley, in which the Court acknowledged that solitary confinement can lead to madness and suicide, to modern studies by psychologists and penologists. He cited a litany of possible side effects to prolonged isolation, including anxiety, panic, withdrawal, hallucinations, and self-mutilation.

“Research still confirms what this Court suggested over a century ago: Years on end of near-total isolation exacts a terrible price,” he summarized.

Kennedy’s critique of solitary confinement in Davis came without warning or fanfare. Davis was not a case about solitary confinement at all; the Court had accepted it to address peremptory challenges during jury selection. But the length and condition of Ayala’s imprisonment appears to have caught Kennedy’s attention. Towards the end of oral arguments on March 3, he briefly quizzed Anthony Dain, Ayala’s lawyer, for more details.

“This crime was, what, 30 years ago and the trial [was] 26 years ago? Has he spent time in solitary confinement, and if so, how much?” Kennedy asked.

“He has spent his entire time in what’s called administrative segregation. When I visit him, I visit him through glass and wire bars,” Dain replied. When the justice inquired about the difference between solitary confinement and administrative segregation, Dain said his client spent 23 hours in his cell and was allowed one hour of activity.

“One hour,” Kennedy muttered in response as Dain’s time expired.

Solitary confinement is a new battleground for the Court’s second-longest serving justice, but not a surprising one. Few justices on the Supreme Court have wielded the Eighth Amendment as expansively as Anthony Kennedy. On the death penalty, he wrote the majority opinions in Roper v. Simmons, which forbade the execution of juvenile offenders, and Kennedy v. Louisiana, which struck down death sentences for non-homicide crimes. In 2011, he joined with the Court’s liberal wing in Brown v. Plata to uphold a landmark federal-court order requiring California to reduce prison overcrowding. Kennedy also provided the fifth vote in Miller v. Alabama, in which the Court struck down mandatory life-without-parole sentences for juveniles in 2012. As the court’s swing vote, he carries tremendous power to shape cases to form majorities.

Although Kennedy addressed solitary confinement specifically, he also invoked mass incarceration in general and the national debate surrounding it. “There are indications of a new and growing awareness in the broader public of the subject of corrections and of solitary confinement in particular,” he noted. Perhaps hoping to reach this broader public, Kennedy wrote without dense legal jargon and cited examples ranging from Kalief Browder to Dr. Manette, the imprisoned father of Lucie in Charles Dickens’s A Tale of Two Cities. “Even Manette, while imprisoned, had a work bench and tools to make shoes, a type of diversion no doubt denied many of today’s inmates,” he observed.

But Kennedy’s concurrence also seemed to be directed toward the American legal community, whose disengagement from prison issues he has previously lamented. “In law school, I never heard about corrections,” he told a congressional hearing on March 23, two weeks after the Davis oral arguments. “Lawyers are fascinated with the guilt/innocence adjudication process. Once [it] is over, we have no interest in corrections. Doctors and psychiatrists know more about the corrections system than we do.”

Although no one realized it at the time, his brief soliloquy on the crisis of “total incarceration” in March was a preview of today’s concurrence. “Too often, discussion in the legal academy and among practitioners concentrates simply on the adjudication of guilt or innocence,” Kennedy wrote. “Too easily ignored is the question is the question of what comes next. Prisoners are shut away—out of sight, out of mind.” Consideration of these issues, he stated, “is needed.”

To that end, Kennedy all but urged the legal community to bring a solitary-confinement case before the Supreme Court as soon as possible. With the proper case, he wrote, “the judiciary may be required, within its proper jurisdiction and authority, to determine whether workable alternative systems for long-term incarceration exist, and, if so, whether a correctional system should be required to adopt them.”

If the other members of the Court share Kennedy’s viewpoint, they did not disclose it. None of the other justices joined his concurrence, and only Justice Clarence Thomas acknowledged it at all. In a one-paragraph response, Thomas evoked the memory of Ayala’s three victims to dismiss Kennedy’s critiques. “I write separately only to point out ... that the accommodations in which Ayala is housed are a far sight more spacious than those in which [his victims] now rest,” he wrote. “And, given that his victims were all 31 years of age or younger, Ayala will soon have had as much or more time to enjoy those accommodations as his victims had time to enjoy this Earth.”

But for Kennedy, the problems with solitary confinement went far beyond any one case, including this one. “Over 150 years ago, Dostoyevsky wrote, ‘The degree of civilization in a society can be judged by entering its prisons,’” the justice concluded. “There is truth to this in our own time.”