6.4 SCL

Finally, let me turn to the SCL (Substrate-Catalyst-Link) system (McMullin & Varela, 1997), which is a recent re-implementation of the original computer model of autopoiesis (Varela et al., 1974). It specifically corrected a defect in the original published description of the model, establishing that a previously undocumented interaction (chain inhibition) was in fact essential to the autopoietic phenomenology of the model. In any case, the experiments reported both in the original paper and in this re-implementation were confined strictly to examination of a single putatively autopoietic system, spatially separated from any other. In this case, such an entity can indeed exhibit a self-sustaining network, with a clearly recognizable spatial structure or boundary which is both maintained by, and in turn confines, the reaction network.

It is then interesting to enquire how this behaviour is affected if two such entities are placed in close spatial proximity, as envisaged in the heuristic test described above. (Since the SCL system is deliberately simplified there is, in fact, only one possible self sustaining reaction network; therefore the multiple instances are necessarily instances of the same network).

Some (unpublished) experiments have been conducted along these lines. The outcome is, at best, inconclusive. The details are quite technical and only an outline will be presented here.

It should first be noted that the self sustaining agents in this system are quite unstable, even in isolation (see the example runs in McMullin & Varela 1997), which makes any interpretation of results quite difficult. However, this instability does seem to be exacerbated when multiple agents are in close proximity. In particular, the chain inhibition interaction, mentioned earlier as crucial to the operation of an isolated agent, turns out to also have an unintended side effect of inhibiting maintenance of the bounding membrane when two membranes are adjacent to each other. This means that, if anything, adjacent agents tend positively to merge rather than to maintain their individuality.

On balance then, I would say that the self sustaining agents in the SCL system do not pass my suggested heuristic test for ``full'' autopoiesis. This must be considered at least a little controversial given that the model was designed explicitly to illustrate autopoietic organization (albeit in minimal form).

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Timestamp: 2003-03-28