A new criterion for demarcating life from non-life.

Criteria for demarcating life from non-life are important for deciding whether new candidate systems, either discovered extraterrestrially or constructed in the laboratory, are genuinely alive or not. They are also important for understanding the origin of life and its evolution. Current criteria are either too restrictive or too extensive. The new criterion proposed here poses that a system is living when it is capable of utilizing active causation, at evolutionary or behavioural timescales. Active causation is produced when the organism uses an estimate of its own Darwinian fitness to modulate the variance of stochasticity that drives hereditary or behavioural changes. The changes are subsequently fed back to the fitness estimate and used in the next cycle of a feedback loop. The ability to use a self-estimated fitness in this way is an evolved property of the organism, and the way in which fitness is estimated is therefore controlled and stabilized by Darwinian evolution. The hereditary and behavioural trajectories resulting from this mechanism combine predictability with unpredictability, and the mechanism produces a form of self-directed agency in living organisms that is absent from non-living systems.