the subject destined to dominate his research for over forty years: the mechanism of inhibitory synaptic transmission. After completing his DPhil in 1929, he joined Sherrington’s research group, and developed a technical improvement of the torsion myograph in preparation for a collaboration concerned with research on the flexion reflex and inhibition. These experiments were to see the last flowering of Sherrington’s scientific genius at the age of seventy-five. The work on the ipsilateral spinal flexion reflex introduced Eccles to the technique of stimulating nerves first with just a threshold conditioning volley, then at later intervals with a subsequent test volley in order to tease out the time course of the central excitatory and inhibitory states. This approach, when applied to the mechanism of transmission in the spinal cord, gave a very precise measure of the time course of the central excitatory and inhibitory states, or, as we now know, the excitatory and inhibitory postsynaptic potentials. This was shown by Eccles and his colleagues some twenty years later, when they made the first intracellular recordings of postsynaptic potentials in motoneurones. Subsequent studies of inhibitory synaptic transmission using intracellular electrodes were carried out by Eccles and his colleagues at successively higher levels of the central nervous system. These provided a functional microanatomy of the synaptic connections to be found in the cerebellum, the thalamus and the hippocampus. In this way Eccles completed the research programme described by Sherrington in The Integrative Action of the Nervous System half a century earlier.
Eccles’s interest in themind–brain problem
Eccles had entered the field of neuroscience as a result of an inspirational experience he had had at the age of eighteen that changed his life and aroused in him an intense interest in the mind–brain problem.12 In the 1970s, evidently stimulated by the work done by R. W. Sperry and his colleagues in the 1960s on the results of hemispherectomy, he turned at last to these philosophical questions of his youth, first in The Self and its Brain (1977), a book he wrote with Karl Popper, and subsequently in his Gifford Lectures of 1977–8, published in 1984 as The Human Mystery. He opened his Gifford Lectures with a handsome tribute to Sherrington’s Gifford Lectures forty years earlier. Eccles remarked that the general theme of Man on his Nature had been the defence of a form of dualism – a doctrine that was, by the 1970s, antipathetic to established philosophy. Nevertheless, it was a doctrine that Eccles deeply admired and, moreover, believed to have been given experimental confirmation by Kornhuber’s work on the electrical potential generated in the cerebral cortex prior to performing an intentional action and by Sperry’s work on split-brain patients. Hence, he aimed, in his own Gifford Lectures, to defend Sherrington’s conception, to ‘define the mind–brain problem more starkly’,13 and to bring to bear on the problem these most recent findings in neuroscience.
Popper’s influence
The general framework for Eccles’s reflections was furnished by Popper’s revival of a misconceived idea of the great nineteenth-century mathematical logician Gottlob Frege.14 Frege distinguished between the perceptible ‘outer world’ of physical objects, the private ‘inner world’ of mental entities, and a ‘third realm’ of thoughts (propositions) that are imperceptible by the senses, but nevertheless public and shareable. Popper followed suit, distinguishing between World 1 of physical things, World 2 of mental things, and World 3 of thoughts, theorems, theories and other abstracta. The conception is confused, since although we distinguish material objects from mental states, and both from propositions or theorems, these do not collectively constitute ‘worlds’ in any sense whatsoever. Furthermore, neither mental states nor propositions are denizens of a distinct ‘world’. There is only one world, which is described by specifying whatever is (contingently) the case. We do indeed talk of people’s mental states of cheerfulness or depression, or of their having toothache. But this does not imply that cheerfulness, depression or toothache are peculiar mental entities that exist in an ‘inner world’. These nominals (‘cheerfulness’, ‘depression’, ‘toothache’) merely provide an indirect way of talking of people being cheerful or depressed and of their tooth’s hurting – it introduces no new entities, merely new ways of talking about existing entities (e.g. about human beings and how things are with them). Similarly, we talk of propositions, theorems and other abstracta – but this too only appears to introduce new entities, and is really no more than a convenient way of talking about what is or might be said, asserted, or proved, etc. There is absolutely no need to succumb to Platonism and conjure new entities into existence and new worlds for them to inhabit. All talk of expressions standing for ‘abstract entities’ is a misleading way of saying that expressions that look as if they stand for concrete entities do not do so at all, but rather fulfil quite different functions. To be sure, this does not mean that there are no mental states, no cheerfulness, depression or anxiety, or that there are no propositions, no theories or theorems. On the contrary, it means that there are – only they are not kinds of entities.
Popper’s three-world doctrine impressed Eccles, and he formulated his dualism in terms of it. World 1, the material world of the cosmos, he declared, consists of mere material things and of beings that enjoy mental states. The latter, being a subset of the entities in World 1, he refers to collectively as ‘World 1 M’. This ‘world’ stands in reciprocal causal interaction with World 2 by means of what he terms ‘the liaison brain’ (HM 211).
The impact on Eccles of Kornhuber’s research on readiness potential
Research done by Kornhuber and his colleagues (see §1.6.1) on changes in electrical potential antecedent to a voluntary movement had revealed that the so-called readiness potential began up to 800 milliseconds before the onset of the muscle action potential, and led to a sharper potential, the pre-motion positivity, beginning at 80–90 milliseconds prior to the movement. The patterns of neuronal discharges eventually project to the appropriate pyramidal cells of the motor cortex and synaptically excite them to discharge, so generating the motor potential (a localized negative wave) just preceding the motor pyramidal cell discharge that initiates the movement. The question on which Kornhuber’s research seemed to throw light was: ‘How can willing of a muscular movement set in train neuronal events that lead to the discharge of pyramidal cells of the motor cortex and so to the activation of the neuronal pathways that lead to the muscle contraction?’ (HM 214). It is striking that Eccles took these discoveries to betoken empirical confirmation of mind–brain interaction of a kind (but in a different location) that had been envisaged by Descartes. He argued as follows:
What is happening in my brain at a time when the willed action is in the process of being carried out? It can be presumed that during the readiness potential there is a developing specificity of the patterned impulse discharges in neurons so that eventually there are activated the pyramidal cells in the correct motor cortical areas for bringing about the required movement. The readiness potential can be regarded as the neuronal counterpart of the voluntary intention. The surprising feature of the readiness potential is its very wide extent and gradual build up. Apparently, at the stage of willing a movement, there is a very wide influence of the self-conscious mind on the pattern of module operation. Eventually this immense neuronal activity is moulded and directed so that it concentrates onto the pyramidal cells in the proper zones of the motor cortex for carrying out the required movement. The duration of the readiness potential indicates that the sequential activity of the large numbers of modules is involved in the long incubation time required for the self-conscious mind to evoke discharges from the motor pyramidal cells … It is a sign that the action of the self-conscious mind on the brain is not of demanding strength. We may regard it as being more tentative and subtle, and as requiring time to build up patterns of activity that may be modified as they develop.(HM 217)
Cartesian problems recapitulated:
(1) Interaction
So, Eccles conceived of what he called ‘the dualist-interactionist hypothesis’ as helping to ‘resolve and redefine the problem of accounting for the long duration of the readiness potential
