Hartley, David (1705–1757)

2. Physiological philosophy

Hartley is best remembered for his Observations on Man, His Frame, His Duty, and His Expectations, published in 1749 – a two-volume philosophical tour de force that systematized views set out in his earlier The Progress of Happiness Deduced From Reason (1734) and Conjecturae quaedam de sensu, motu, et idearum generatione (1746). The Observations on Man offered a comprehensive vision of the individual considered both as an earthly being and in regard to a future state. Emphasizing the view that all knowledge derives from experience, it drew heavily upon the empiricist theory of mental operations explicated in John Locke’s Essay Concerning Human Understanding (1690; see Locke, J.). It also absorbed the innovative associationist utilitarianism of the Revd John Gay’s Preliminary Dissertation Concerning the Fundamental Principle of Virtue or Morality (1731), which set out a pleasure and pain psychology as the key to the formation of opinions and to the philosophy of action. Like Locke and Gay, Hartley sought to refute nativist theories of cognition and morality (see Nativism), insisting that complex ideas were built up from simple imputs by repeated combinations of what Hartley called the ‘sensations of the soul’. Hartley thus grounded thoughts and values upon the Lockean principle of the association of ideas.

Unlike Locke and Gay, however, Hartley aimed to set these epistemological and psychological observations upon concrete physical foundations – the anatomy of the nervous system and the physiology of ‘motions excited in the brain’. For this he drew upon the theory of sensation suggested in the ‘Queries’ to Isaac Newton’s Opticks (1704; see Newton, I.). Newton had shown how light vibrated in a medium; such vibrations had an impact upon the retina; having impinged upon the eye, Hartley argued, these corpuscular motions set off further vibrating waves that passed along the nerves to the brain. The Lockean notion of the association of ideas was thus visualized and made material by Hartley in terms of reiterated vibrations in the white medullary matter of the brain and spinal cord, which resulted in lasting traces or vestiges that served as the physical substrate of complex ideas, memory and dispositions. Unlike the French philosophes like La Mettrie, Hartley framed his materialist physiological psychology in terms of a Christian natural theology. In his view, materialism was not the slippery slope to atheism, precisely because it was the Christian God who, in His Wisdom, had endowed matter with all its powers and potentialities. The necessitarianism entailed by materialism was, in Hartley’s opinion, the finest guarantee of the universal operation of cause and effect, hence of the uniformity of nature, and so of the boundless power of God.

As befitted a medical man, the first volume of Observations on Man explored major aspects of neurophysiology, discussing the human mind and appetites in terms of the evolution of complex ideas and habits from elementary sensations. Hartley demonstrated the formation of mental associations on the basis of the vibrations of particles in the nervous system which persisted in the form of the more minute ‘vibratiuncles’, which in turn provided the physical basis for memory, regarded in something of a rather Hobbesian way as decaying sense (see Hobbes, T.). Individual chapters explored the mental physics of feeling, taste, smell, seeing and hearing. The second volume extended the system to account for morality and the individual’s prospects in a future state.

Unlike Stephen Hales, Hartley was less an experimenter than a systematizer, offering a comprehensive framework for interpreting the phenomena of life and mind. Among his followers, the Observations on Man came to be seen as the fountainhead of key biological, psychological and social doctrines. Hartley’s tenets provided the framework for the associationist heritage in psychology – in particular, learning theory. His conjectures concerning the physiology of the nervous system offered suggestions for sensory-motor theories later influential in neurophysiology, and for the experimental localization of brain functions. Hartleian notions are the distant ancestors of Pavlovian notions of conditioned reflexes.