 Crathorn endeavored elsewhere to gather at least a few certitudes, since one could doubt whether the a priori principle that God would never deceive us is itself certain. To rebut the skeptic, he turns back to Augustine’s version of the cogito-argument to prove that we can at least be certain of our own mental activity, for if one were to doubt a proposition such as ‘I am’, it would follow that he exists, since he who does not exist does not doubt. Hence, no one can be in doubt concerning the proposition, ‘I am’ (I Sent., q. 1, concl. 14; translated in Tachau 1988, p. 273).