Custom and Habit in Physiology and the Science of Human Nature in the British Enlightenment


In: Early Science and Medicine

In this paper I show how what came to be known as “the double law of habit,” first formulated by Joseph Butler in a discussion of moral psychology in 1736, was taken up and developed by medical physiologists William Porterfield, Robert Whytt, and William Cullen as they disputed fundamental questions regarding the influence of the mind on the body, the possibility of unconscious mental processes, and the nature and extent of voluntary action. The paper shows, on a particular topic, the overlap between eighteenth-century philosophical writings on the science of human nature on the one hand, and medical writings and lectures in physiology on the other. Other early modern writers discussed in the paper include René Descartes, Herman Boerhaave and David Hume.


  • 6

     Descartes, Philosophical Writings, 1: 314-5. La Description du Corps Humain was published by Clerselier in 1664 along with the Traite de l’Homme; see Descartes, Oeuvres, 1:217-90. It was written in 1647-8.

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  • 7

     My translation. See Descartes, Oeuvres, 7: 160. The original Latin that is translated here reads “immediate conscii sumus” using the equivalent to our modern word ‘conscious.’ Descartes, Philosophical Writings, 2: 113 uses the English word “aware,” which loses the significance of Descartes’ use of “conscii.”

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  • 22

     Boerhaave, Academical Lectures, 401.24.

  • 25

     Butler, Analogy of Religion, 84.

  • 26

     Butler, Analogy of Religion, 83.

  • 27

     Butler, Analogy of Religion, 82-4.

  • 31

     Wright, “Butler and Hume,” 105-18.

  • 33

     Hume, Human Nature, 423.

  • 35

     Hume, Human Nature, 418-9.

  • 36

     Hume, Human Nature, 418.

  • 37

     Hume, Human Nature, 581, 583.

  • 38

     Hume, Human Nature, 603. Compare ibid., 581.

  • 40

     William Porterfield, “An Essay concerning the Motions of our Eyes,” in Medical Essays and Observations: Revised and Published by a Society in Edinburgh , 4 Vols. (Edinburgh, 1737-8), Vol. 3, pp. 160-263, Vol. 4, pp. 124-294.

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  • 41

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part I, Of their External Motions” in Medical Essays and Observations, vol. 3, 187.

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  • 42

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part I, Of their External Motions” in Medical Essays and Observations, vol. 3, 189.

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  • 43

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part I, Of their External Motions” in Medical Essays and Observations, vol. 3, 261.

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  • 44

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part II, Of their Internal Motions,” Medical Essays and Observations, vol. 4, 124-294, at 213-4. These include the uniform motions of the eyes, the change in shape of the crystalline lens of the eye when we change our distance from the object, and the closing of our eyelids when an object has been thrust in our face. This last, as we have seen above, had been identified as an automatic reflex action by both Descartes and Boerhaave.

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  • 45

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part II, Of their Internal Motions,” Medical Essays and Observations, vol. 4, 213-4; my emphasis.

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  • 46

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part II, Of their Internal Motions,” Medical Essays and Observations, vol. 4, 216.

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  • 47

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part II, Of their Internal Motions,” Medical Essays and Observations, vol. 4, 217.

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  • 48

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part II, Of their Internal Motions,” Medical Essays and Observations, vol. 4, 226.

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  • 49

     William Porterfield, “An Essay concerning the Motions of our Eyes, Part II, Of their Internal Motions,” Medical Essays and Observations, vol. 4, 226-7.

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  • 51

     Porterfield, “Motions of our Eyes, Part I,” 257-8.

  • 53

     Porterfield, “Motions of our Eyes, Part I,” 260.

  • 54

     Porterfield, “Motions of our Eyes, Part II,” 225-6.

  • 86

     See Wright, “Metaphysics and Physiology,” 275 and John P. Wright, “The Embodied Soul in Late Seventeenth-Century French Physiology,” Canadian Bulletin for the History of Medicine, 8 (1991), 21-42, esp. 29ff.

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