Andreas Vesalius (1515-1564) on animal cognition.

Until well in the 19th century, the Aristotelian concept of the scala naturae (ladder of nature) was the most common biological theory among Western scientists. It dictated that only humans possessed a rational soul that provided the ability to reason and reflect. Michel Eyquem de Montaigne (1533-1592) was the first philosopher influential enough to lastingly posit that animals are cognitive creatures. His view stirred a fierce controversy, with René Descartes (1596-1650) leading among his many adversaries. Only after it became accepted that animals and humans alike have cognitive abilities, did the research on the influence of conscious awareness and intention on the behavior of an animal become possible in the 20th century. We found the anatomist Andreas Vesalius (1515-1564) to have already rejected the Aristotelian view on the lack of the rational soul in animals in his 1543 opus magnum De Humani Corporis Fabrica Libri Septem. His observation "that there is a difference in size according to the amount of reason that they seem to possess: man's brain is the largest, followed by the ape's, the dog's, and so on, corresponding to the amount of rational force that we deduce each animal to have" resonated some 330 years later when Darwin concluded that "the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind." We conclude that Vesalius was instrumental in breaking with two millenniums of dominance of the concept of lack of animal cognition.