Philip J. van der Eijk, "The Heart, the Brain, the Blood and the pneuma: Hippocrates, Diocles and Aristotle on the Location of Cognitive Processes," in Medicine and Philosophy in Classical Antiquity: Doctors and Philosophers on Nature, Soul, Health and Disease (Cambridge University Press, 2005), pp. 131–132 et passim.[Enllaç no actiu]ISBN 0-521-81800-1
Cleanthes, wishing to give more explicit meaning to Zeno's 'creative fire', had been the first to hit upon the term pneuma, or 'spirit', to describe it. Like fire, this intelligent 'spirit' was imagined as a tenuous substance akin to a current of air or breath, but essentially possessing the quality of warmth; it was immanent in the universe as God, and in man as the soul and life-giving principle.Marcus Aurelius. Meditations. Londres: Penguin Books, 1964, p. 25. ISBN 0-14044140-9.
