Abstract

It is crucial, first of all, to stress the importance Churchland attaches to the idea that the neural networks whose assemblages he holds to be “engines of reason” must be recurrent. Non-recurrent networks, of the sort best known among philosophers, simply discover patterns in input data presented to them as sets of features. The learning capacities of such networks, extensively discussed since the publication of Rumelhart and McClelland et al., are indeed impressive; and Churchland describes them clearly and gracefully as preparation for introducing recurrent networks. Now, the importance of recurrence as a feature of the networks Churchland hypothesizes as forming the basis of cognitive activity is well motivated. A cognitive system that was an assembly of non-recurrent networks would be, in essence, a stimulus-response machine of the sort that early behaviourists took themselves to be studying when they examined the mind/brain. Such a system could learn to find novel patterns in data. However, it is empirically evident that human— and some non-human—cognitive capacities go well beyond this. People dream, imagine scenarios that never existed, reconceptualize their perceptions, and theorize. All these activities seem to require not merely inferring patterns from data, but reconceiving the nature of the data itself in light of the inferential structures built through learning. For this to be possible, the data cannot be “clamped,” as they are in the case of non-recurrent networks. Hence the importance of recurrence. A recurrent network can be defined as one which takes some or all of its own output and then treats that output as a source of input. This allows for the recognition of meta-patterns. Furthermore, if some such networks are feeding their output as input to other networks with which they are linked, the possibility of reasoning and conceptualizing by analogy arises; and this ability, as Churchland argues, seems to be an essential aspect of genuine creativity in science, art, and the continuously ongoing re-design of the social order. The actual existence of simulated recurrent networks, and the capacities they demonstrate, constitute an impressive possibility argument that a system with enormous cognitive plasticity and behavioural flexibility could be built out of parallel distributed processing networks, so long as at least many of these networks were recurrent. The importance of this possibility argument should not be under-emphasized, but it of course invites an obvious question: how good are our grounds for supposing that the possibility in question is actually realized in the biological domain?