Historically, when philosophers and scientists define a concept, the aim is to provide necessary and sufficient conditions. These theoretical definitions (also called real, ideal or philosophical definitions) are often impractical or fragile as they can be challenged with a single imagined counterexample. A classic case is the definition of “bachelors” as “unmarried males.” It is trivial to find examples that fit this definition without intuitively being bachelors: male dogs, baby boys, widowers, etc. Similarly, for any definition of life, one can either show living cases that are left out of the definition or non-living cases that are included by it. Life is organized, but so are geological formations. Life processes energy, but so does fire. Life evolves using complex biochemistry, but so do prions. Life is self-sustaining, but parasites are not. Life is at thermodynamic disequilibrium, but so is much else. As we’ll see shortly, perhaps theoretical definitions are too rigid a standard. The real world is far too complex for limited criteria to decide every marginal case.