Evaluation of Example 1: Descartes on Animal Spirits

Passage:

Jan Swammerdam's Frog Heart Experiment — Adapted from John Swammerdam, 'The Book of Nature' trans. Thomas Flloyd (New York: Arno Press, 1976), II: 126 also in Matthew Cobb, 'Exorcizing the Animal Spirits,' 'Nature Reviews: Neuroscience' 3 no. 5 (May 2002), 398. doi:10.1038/nrn806. “[W]hat above all is here worthy of observation, is the generation of the animal spirits, which are like a very subtle wind, or rather a very pure and vivid flame which, continually ascending in great abundance from the heart to the brain, thence penetrates through the nerves into the muscles, and gives motion to all the members; so that to account for other parts of the blood which, as most agitated and penetrating, are the fittest to compose these spirits, proceeding toward the brain, it is not necessary to suppose any other cause …”

Rene Descartes, Discourse on Method, (1637 London: J.M. Dent, 1912), 43.

Analysis:

Descartes argues that parts of the blood compose “animal spirits” which the heart sends to the brain, and the brain conveys via the nerves to the muscles. He claims this is the only cause of bodily motion. The fallacy of false cause occurs as the generation of such spirits cannot be observed and no explanation is provided for the generation of muscular movement from the supposed spiritual parts of blood infusing the muscle.

Descartes proposal of the hydraulic theory of muscular movement was the conjecture that the pressure of “animal spirits” from ventricles in the brain were the cause of the expansion of muscles and their consequent contractions. Jan Swammerdam unwittingly demonstrated Descartes' theory was mistaken with a frog heart placed in an air-tight glass syringe with an drop of water suspended in the glass needle. As the heart spontaneously contracted and dilated the drop of water d dropped and rose in tandem as shown in the accompanying image — disproving Descartes' ballonist theory of muscular movement.