"Life is a self-sustained chemical system capable of undergoing Darwinian evolution."

That's the so-called "NASA definition of life."

Or there's Russian-born geneticist Edward Trifonov's take: Life is "self-reproduction with variations."

Or there's "Life is an expected, collectively self-organized property of catalytic polymers." Or "Life is a metabolic network within a boundary."

It's said that, "There are as many definitions of life as there are people trying to define it." And yet, none of those definitions is quite right.

Science writer Carl Zimmer says that's strange behavior for scientists: "It is as if astronomers kept coming up with new ways to define stars."

Plus: Linguist Ben Zimmer (no relation to Carl*) on what it means to be human. Or, at least, as much as we can tell about what it means to be human by looking at Dr. Fill, the artificial intelligence that just won a national crossword puzzle tournament.

*No relation beyond that they're brothers, I mean.

GUESTS:

• Ben Zimmer - A linguist, lexicographer, and the language columnist for The Wall Street Journal
• Carl Zimmer - The science columnist for The New York Times; his new book is Life's Edge: The Search for What It Means To Be Alive

Join the conversation on Facebook and Twitter.

Colin McEnroe and Cat Pastor contributed to this show.

Support the show: http://www.wnpr.org/donate

See omnystudio.com/listener for privacy information.