International Journal of Astrobiology

Alien life: how would we know? 1

Margaret A. Boden a1
a1 Centre for Cognitive Science, University of Sussex, Sussex, UK e-mail:

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To recognize alien life, we would have to be clear about the defining criteria of ‘life’. Metabolism – in other words, biochemical fine-tuning – is one of these criteria. Three senses of metabolism are distinguished. The weakest allows strong artificial life (A-life): virtual creatures having physical existence in computer electronics, but not bodies, are classed as ‘alive’. The second excludes strong A-life but allows that some non-biochemical A-life robots could be classed as alive. The third, which stresses the body's self-production by energy budgeting and self-equilibrating energy exchanges of some (necessary) complexity, excludes both strong A-life and living non-biochemical robots.

(Received June 1 2003)
(Accepted June 22 2003)

Key Words: autopoiesis; life; A-life; artificial life; metabolism; self-organization.


1 This paper is adapted from Boden, M.A. (1999). Is metabolism necessary? Brit. J. Phil. Sci. 50, 231–248.