Scientists don’t really agree on a definition for life. We may recognize life instinctively most of the time, but any time we try to nail it down with set criteria, some stubborn counterexample spoils the effort. Still, can we really search for life on other worlds, or understand the earliest stages of life on this planet, if we don’t know what to look for? On this episode, Steven Strogatz speaks with Robert Hazen, a mineralogist, astrobiologist and senior staff scientist at the Carnegie Institution’s Earth and Planets Laboratory, along with Sheref Mansy, professor of chemistry at the University of Alberta, to learn more about how new taxonomies and a “cellular Turing test” might help us answer this essential question.

“The Joy of Why” is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine. “The Joy of Why” is produced by Susan Valot and Polly Stryker. Our editors are John Rennie and Thomas Lin, with support by Matt Carlstrom, Annie Melchor and Leila Sloman. Our theme music was composed by Richie Johnson. Our logo is by Jackie King, and artwork for the episodes is by Michael Driver and Samuel Velasco. Our host is Steven Strogatz. If you have any questions or comments for us, please email us at quanta@simonsfoundation.org.