The treatise De spiritu of the Corpus Aristotelicum deserves better treatment than it has received since W. Jaeger in his 1913 article rejected its authenticity and dated it one hundred years after Aristotle. In this paper the authors argue that De spiritu defends purely Aristotelian viewpoints against persons like Plato and Empedocles, who held respiration to be the most important vital process. Most of the De spiritu is directed against the pneuma doctrine of Plato's Timaeus. The 'Aristogenes' mentioned in De spiritu 2 is either Plato 'the son of Ariston' or a contemporary pupil of Plato and Aristotle. © 2007 Brill Academic Publishers.