Writing in Dublin in 1944, Erwin Schrödinger sought the source of order in biological systems. Given the recent radiation mutagenic evidence on the target size of a gene showing that a gene had at most a few thousand atoms, Schrödinger argued that the familiar order due to square root N fluctuations around an equilibrium was insufficient because the fluctuations were too large to account for the hereditary order seen in biology. He argued that quantum mechanics, via stable chemical bonds, was essential for that order. Then he made his brilliant leap. Noting that a periodic crystal could not “say” very much, he opted for genes as aperiodic crystals which, via the aperiodicity, would carry a microcode specifying the ontogeny of the organism. It was a mere two decades until the structure of the aperiodic double helix of DNA and much of the genetic code were known.

But did Schrödinger's book, What Is Life? actually answer his core question? I think not, and the aim of this chapter is to propose a different definition, one concerning what I call an “autonomous agent,” that may have stumbled on an adequate definition of life. I will not insist that I have succeeded, but at a minimum the definition leads in many useful and unexpected directions with import for physics, chemistry, biology, and beyond.