This is a scarce Cartesian polemic: Pars I (1653): 64 pages Pars II (1654): about 1,040 pages Henricus Regius was one of the earliest and most visible defenders of René Descartes at a Dutch university. He was a professor of medicine and philosophy at Utrecht and a close associate of Descartes in the 1640s. This 1653 book is a polemical defense of Cartesian method, written against Jacobus Revius (1586–1658), a Calvinist theologian and poet who had attacked Descartes’ ideas as heretical and dangerous to theology. The Assertio responds to Revius’ theological critique of Descartes, especially regarding the nature of substance, motion, and the soul.