Vivisection was a very common form of experimentation and demonstration in medical colleges of the time - and in various forms, up to the present. Whether Descates himself conducted any such lectures using dogs has been the subject of debate, but he was a practicing physician, so he must have at least attended those lectures. He certainly didn't invent or initiate them, but he was famous, and his apologetics did help to legitimize vivisection as sound scientific practice.
Descartes famously thought that animals were merely ‘mechanisms’ or ‘automata’ – basically, complex physical machines without experiences – and that as a result, they were the same type of thing as less complex machines like cuckoo clocks or watches. He believed this because he thought that thoughts and minds are properties of an immaterial soul; thus, humans have subjective experience only because they have immaterial souls inhering in their physical bodies. However animals, reasoned Descartes, show no signs of being inhabited by rational souls: they don’t speak or philosophise, and so (as far as we can tell) they lack souls, and minds.
He bent some little way to accord animals sensation and emotion, but still considered it legitimate for humans to use them like objects. — Vera Mont
In 1641 Descartes published the Meditations on First Philosophy, in Which Is Proved the Existence of God and the Immortality of the Soul. Written in Latin and dedicated to the Jesuit professors at the Sorbonne in Paris, the work includes critical responses by several eminent thinkershttps://www.britannica.com/biography/Rene-Descartes/Meditations
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