The French philosopher René Descartes (1596–1650) has been one of the most influential figures in the history of philosophy and the sciences. The long-prevailing view of a dichotomy between mind and body in western thinking derives from Descartes’ differentiation between the laws governing the external physical world and the private facts of the subjectively experienced mental life (res extensa and res cogitans). In her introduction to the latest Finnish edition of his writings, Alanen (2001) describes Descartes’ work as an ambitious endeavour to establish a scientific explanation of the world ruled by natural laws and to lay down principles of thought that would lead to a certainty beyond question. Descartes’ influence has been greatest in the field of philosophy of the mind. His achievements in mathematics and optics were also considerable, as was his thinking in moral philosophy, but his physiological investigations did not lead to any definitive findings.