Sean Carroll in Preposterous Universe:
Someday, most likely, we will encounter life that is not as we know it. We might find it elsewhere in the universe, we might find it right here on Earth, or we might make it ourselves in a lab. Will we know it when we see it? “Life” isn’t a simple unified concept, but rather a collection of a number of life-like properties. I talk with astrobiologist Stuart Bartlett, who (in collaboration with Michael Wong) has proposed a new way of thinking about life based on four pillars: dissipation, autocatalysis, homeostasis, and learning. Their framework may or may not become the standard picture, but it provides a useful way of thinking about what we expect life to be.