  The ultimate goal of astrobiology is to determine the distribution and
diversity of life in the universe. But as the word "biosignature" suggests,
what will be detected is not life itself, but an observation implicating a
particular process associated with living systems. Technical constraints and
our limited access to other worlds suggest we are more likely to detect an
out-of-equilibrium suite of gasses than a writhing octopus. Yet, anything short
of a writhing octopus will raise skepticism among astrobiologists about what
has been detected. Resolving that skepticism requires a theory to delineate
processes due to life and those due solely to abiotic mechanisms. This poses an
existential question for the endeavor of life detection: How do astrobiologists
plan to detect life via features shared between non-living and living systems?
We argue that you cannot without an underlying theory of life. We illustrate
this by analyzing the hypothetical detection of an "Earth 2.0" exoplanet. In
the absence of a theory of life, we argue the community should focus on
identifying unambiguous features of life via four areas of active research:
understanding the principles of life on Earth, building life in the lab,
detecting life in the solar system and searching for technosignatures.
Ultimately, we ask, what exactly do astrobiologists hope to learn by searching
for life?
