Vladimir Putin killed my friend Alexei Navalny this week. There will be a time and place to discuss the politics and how the free world should respond. For now, I want to share a few memories.

The Alexei Navalny I knew was super smart. For me, the sign of a true intellectual is the courage to change your mind. We disagreed about some things he had said in the past, particularly about the Caucasus and Crimea. He listened, and it felt to me he was rethinking some of his earlier statements. But he also pushed back, challenging my commitment to “neoliberalism” in the 1990s. Russia would be better off, he argued, had the West supported social democratic ideas back then. He was right. I changed my mind, too. That’s called learning. He was exceptional at that.

The Alexei I knew was extremely charming. I remember our first meeting at the White House in 2009 when I worked at the National Security Council. My boss at the time, Barack Obama, was known for his charisma. Navalny had Obama-caliber presence. I understood that day why Putin feared him. In a free and fair election, Navalny would have destroyed Putin. Remember that the next time you read a poll in the media about Putin’s popularity.

The Alexei I knew was incredibly funny. When I served as the U.S. ambassador to Russia from 2012-2014, we never had a formal meeting, just one chance encounter at an anniversary dinner for the Moscow Times. Navalny understood that any public meetings with me would just fuel conspiracy stories constantly propagated on Putin’s media channels — that the United States was funding his operation. So, instead, we interacted on Twitter, always in a humorous, playful way.

When I once called Russia a “wild country” after a group of paid Putin agitators accosted me on the street, it felt like the whole world, including many in the United States, berated me for my “undiplomatic” outburst. Not Alexei. He asked on Twitter why I did not just belt them, since I had diplomatic immunity. In another tweet, lampooning Putin’s conspiracy-mongering, Navalny instructed me to meet him at some metro station in the “last wagon” (a common way to meet people during the Soviet days) to do our “secret” business.

The Alexei I knew — and that the world knew — was incredibly brave and firmly committed to his values: fighting Putin’s corruption and trying to liberate his country from totalitarian dictatorship. As a scholar of democratization and a sometimes activist for democracy, I have studied or had the privilege of meeting some of the most courageous freedom fighters in the world. Navalny was one of them — the Mandela of Putin’s Russia. Nelson Mandela survived his three decades of captivity. Navalny tragically did not.

The Alexei I knew was a fierce family man. He was so proud of his daughter, Dasha, when she got into Stanford, where I teach. He and his wife, Yuliya, were just like all the other excited Stanford parents when they dropped Dasha off on campus her first year. And they have watched her grow into a strong, principled, charismatic leader, just like her dad and mom. Of course, Alexei had to watch from afar.

In fact, well before he decided to go back to Russia, it seemed to me his deepest anxiety was not about enduring torture in Putin’s gulag or even facing death, but about being an absentee father and husband. By doing what he thought was right for his country, he knew that he was asking his family to sacrifice a lot, too. And today, that sacrifice has grown so much larger.

Navalny dreamed of a free Russia. Barbaric dictators such as Putin can kill men, but they cannot kill ideas. I do not know when, but I am confident that Navalny’s ideas of freedom will outlive Putin’s ideas of tyranny.