ATHENS — If we fail to learn from the past, we do so at our own peril. That was one of the messages from the Chinese artist-activist Ai Weiwei to a gathering of world leaders, academics and activists at the Athens Democracy Forum here last week.

“There’s a potential to totally forget the past, and not to remember those lessons we have learned,” said Mr. Ai, who spoke in support of migrants around the globe. “We have a potential to be very mean, to be hateful, not only to close in but really damage others, and to use some excuse and reason to separate humanity. I think that is part of human nature.”

Mr. Ai, whose recent artwork has been inspired by the plight of refugees, said that all countries needed to unite and assert “global leadership” on the issue of migration, because it was “not going to stop” and might worsen because of environmental disasters, famine and population growth.

Yet a strikingly different response to global migration came from others at the forum, which was convened by The New York Times in cooperation with the city of Athens and the United Nations and is in its sixth year.