In the article on Saint Thomas Aquinas in the Stanford Encyclopedia of Philosophy professor Ralph McInerny writes:
“Thomas accepts from Boethius that it is self-evident that what a thing is and its existing differ (diversum est esse et id quod est).” 
And earlier, in his Boethius and Aquinas, Ralph McInerny had made the following remark:
“That the diversity between esse and id quod est is self-evident is one of the great overlooked claims of De hebdomadibus and of Thomas's commentary on it.” 
↑ Ralph McInerny and John O’Callaghan, “Saint Thomas Aquinas,” The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.). (First published online Mon Jul 12, 1999; substantive revision Wed Sep 30, 2009; accessed August 3, 2011.)
↑ Ralph McInerny, Boethius and Aquinas, (Washington: Catholic University of America Press, 1990), p. xiv.