Suárez and Descartes on the Mode(s) of Union

in a january 1642 letter, rené descartes advises his correspondent—his then-follower, the Utrecht medical professor Henricus Regius—to consistently endorse the view that the human mind is related to its body by means of a "substantial union": Whenever the occasion arises, as much privately as publicly, you ought to profess that you believe a human to be a true ens per se and not per accidens and the mind to be really and substantially united to the body not through position or disposition [situm aut dispositionem], as you say in your last written text, but through a true mode of union [verum modum unionis], as everyone...