Life.

Unfortunately we seem to be no closer to arriving at a satisfactory definition of life. Maturana and Varela's concept of autopoiesis may be adequate for the classification of individual organisms. However, as we are ultimately interested in creating artificial life, or at least in understanding why life exists on Earth, we need to look at a somewhat broader picture which considers the generative processes by which life has emerged. Both evolutionary and ecological considerations will be important for this purpose. We will keep these ideas in mind when developing our artificial models, but specific research questions will be phrased in terms of more specific concepts such as `hierarchical object complexity' and `open-ended evolution'. In Chapter 7 we will revisit the problem of providing an adequate characterisation of the concept of life, in the light of experience gained from building and experimenting with an `artificial life' system (described in Chapters 4-6).