What exactly is life?

How do a bioengineer and a systems biologist view an organism, and how do an ornithologist and an evolutionary ecologist view it? What allows an organism—a conglomeration of cells and the processes occurring between them—to maintain its identity? Since our bodies contain roughly the same number of cells with our own DNA as with foreign ones, should we think of ourselves as a symbiotic community? Our guests will attempt to answer one of the key questions in the philosophy of biology: what is life?