Whytt's theories on the nervous system and its role on movement opposed many of the teachings that were in place in the 18th century. During that time, many physiologists still supported Descartes' theory of movement which hypothesized that muscle contraction was due to the activation of a fluid in the nervous system called . Physiologists, such as Whytt's colleague, Albrecht von Haller, also believed that muscles were capable of action independent of the nerves. Whytt strongly advocated against Descartes' theory, and explicitly denied the concept of animal spirits. Furthermore, he rejected Haller's theory by claiming that movement must depend on interconnecting nerves that lead to the brain or spinal cord.