In light of conceptual difficulties with past and current definitions of life, we present a novel characterisation of the living state based on four pillars: thermodynamic dissipation, autocatalysis, homeostasis and learning. We clarify forms of life by introducing the term ‘lyfe’ to describe any system that performs all four fundamental processes, while ‘life' refers only to living systems as we know them on Earth. We note that many non-living structures exhibit subsets of these properties, and we refer to such systems as ‘sublyfe’. Finally, we review exotic lyfeforms that satisfy the four pillars but differ from lifeforms in distinct ways. We suggest a possible form of lyfe that transduces kinetic energy into its metabolism, a so-called mechanotroph.

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