IN 1890, THE U.S. Supreme Court almost declared the practice of solitary confinement in prisons to be unconstitutional.

In a majority opinion, Justice Samuel Miller — a physician and a lawyer — emphasized the practice’s serious risks to mental health: “A considerable number of the prisoners fell, after even a short confinement, into a semi-fatuous condition, from which it was next to impossible to arouse them, and others became violently insane; others still, committed suicide.” Miller also emphasized solitary confinement’s futility as a punishment: Those “who stood the ordeal better were not generally reformed,” he wrote, “and in most cases did not recover sufficient mental activity to be of any subsequent service to the community.”

More than a century later, the push to reform solitary confinement has resumed. Last month Sen. Richard J. Durbin (D-Ill.) chaired a hearing before the Judiciary subcommittee on the Constitution, civil rights and human rights to reevaluate the practice. Mr. Durbin called solitary confinement “a human rights issue we can’t ignore.” He’s correct.

The United States holds the highest number of prisoners in solitary confinement of any democratic nation. An estimated 25,000 people are kept in federal and state supermax prisons in 44 states, and as many as 80,000 may be kept in some other sort of segregated facility.

The conditions these prisoners endure, as The Post’s Anita Kumar revealed in a recent piece on Virginia’s Red Onion State Prison, are just as Miller described more than a century ago: Most are kept in small cells for 23 hours a day with almost no human contact.

Proponents of the practice insist that sequestering the worst inmates in conditions like these protects prison safety and rehabilitates deviant behavior. Prison safety is a legitimate concern, but solitary confinement doesn’t come close to fixing bad behavior. If anything, it makes it worse. Numerous psychiatric studies show that severe isolation can exacerbate mental illnesses and even cause mental illnesses in prisoners with no previous medical history. Indeed, solitary confinement can impede brain function, cause psychosis and depression, and even lead to suicide. Approximately half of prison suicides occur in solitary confinement.

If prisoners endure, they’re often incapable of functioning in the general population, their social development stunted by their time locked away. The goal of any correctional system should be the exact opposite. Given that solitary confinement costs, on average, three times as much per inmate as regular prison, there’s no justification for the practice in most cases.

As Dr. Atul Gawande asked in a 2009 New Yorker essay: “If prolonged isolation is — as research and experience have confirmed for decades — so objectively horrifying, so intrinsically cruel, how did we end up with a prison system that may subject more of our own citizens to it than any country in history has?” This is a question that demands an answer, and Mr. Durbin is to be commended for spearheading this effort at reform.