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Figure 7.11.
schematic of a chemical computer. source: Pask 1960b, 247, fig. 4.
grow outward from their tips into the liquid: figure 7.12 is a photograph of
a stage in this process. Very simple, but so what? Three points about such
devices need to be understood to appreciate Pask's vision. First, the threads
are
unstable
: they grow in regions of high current density but dissolve back
into solution otherwise. Second, the threads grow
unpredictably
, sprouting
new dendritic branches (which might extend further or dissolve)—“The mo-
ment to moment development of a thread proceeds via a trial process. Slender
branches develop as extensions of the thread in different directions, and most
of these, usually all except the one which points along the path of maximum
current, are abortive” (Pask 1958, 165). Such a system can be seen as conduct-
ing a search through an open-ended space of possibilities, and we can also see
that in Ashby's terms its has the high variety required of a controller: it can
run through an endless list of material configurations (compare the space of
thread geometries with the twenty-five states of the homeostat). Third, as
extensions of the electrodes, the threads themselves influence current densi-
ties in the dish. Thus, the present thread structure helps determine how the
structure will evolve in relation to currents flowing through the electrodes,
and hence the growth of the thread structure exhibits a path dependence in
time: it depends in detail on both the history of inputs through the electrodes
and on the emerging responses of the system to those. The system thus has a
