Mark Kleiman, an intellectual giant in criminal justice and drug policy, died at 68 years old on Sunday due to complications from a kidney transplant, his sister confirmed.

Kleiman, who last worked as a public policy professor at New York University’s Marron Institute, was known for his imaginative approach to policy. He had a knack for breaking through simplified public debates and finding alternative answers to complex problems. As Stanford drug policy expert Keith Humphreys put it, Kleiman “was one of the most creative criminal policy experts of his generation.”

With marijuana legalization, for instance, Kleiman was known for rejecting what he described as a false choice between criminal prohibition and commercial legalization — arguing that there was a middle ground that would end prohibition while preventing the rise of “Big Marijuana,” an entity he, and other experts, feared will market pot irresponsibly just as the alcohol, tobacco, and opioid industries have.

Kleiman also helped research breakthrough approaches for tacking crime and drug misuse. His study with Angela Hawken on Hawaii’s Opportunity Probation with Enforcement (HOPE) program helped demonstrate the principles of “swift, certain, and fair” punishment — a concept that, when properly implemented, uses prison sentences much shorter than those we have today to deter people from criminal behavior, with high success rates. It suggested there was a policy approach that could lead to both less incarceration and less crime.

The last time Kleiman and I talked, he was particularly excited about the preliminary results he had seen from helping implement local versions of a “graduated reentry” system, which he and his colleagues wrote about for Vox, that could ease mass incarceration by releasing inmates back into society with structured opportunities for jobs, housing, and education.

Previously, he helped implement marijuana legalization in Washington state. He also recently participated in New York Gov. Andrew Cuomo’s commission to study cannabis legalization; that commission’s recommendations almost led New York state to legalize marijuana earlier this year.

His book on legal pot with Jon Caulkins and Beau Kilmer, Marijuana Legalization: What Everyone Needs to Know, remains, in my mind, the single best breakdown on the topic.

All of that only scratches the surface of a decades-long career in which Kleiman led discussions on many policy issues, including how to legally classify drugs, the medical legalization of psychedelics, the problems with how the American criminal justice system fights crime, and the politics of crime policy during previous eras. In 2014, Politico included him in a list of 50 people whose ideas are “changing politics.”

But Kleiman was also known for being incredibly kind and helpful. When I started at Vox in 2014, I found myself covering criminal justice and drug policy issues — especially marijuana legalization, which was taking off at the state level — at the national level for the first time in my career. I had a lot of really basic questions.

When I called, Kleiman would always ask how I was doing, making sure things were going well at Vox and inquiring about what I was working on. It was a small thing, but exchanging details about our newest projects made it clear he actually cared about the work; it wasn’t just a transactional interview.

Kleiman would then patiently walk me through all sorts of issues. And when he (rarely) didn’t have an answer or thought I should get multiple perspectives on the topic, he’d readily connect me to his colleagues — sometimes lifting up people early in their careers — for help. Even when we disagreed, he would calmly walk me through his views to help me understand where he was coming from. He became not just a great source but a good friend.

The world has lost an intellectual pillar on criminal justice and drug policy issues. He will be missed.

If you want to see Kleiman in action, I strongly recommend his conversation, from 2014, with Ezra Klein for Vox — a great demonstration of Kleiman’s wit and insight, and one of the best conversations about marijuana legalization to date:

His talk on which drugs should be legalized, and just how legal they should be, is also well worth watching:

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