This point has been argued far more eloquently and accurately by Cardelli and Wagner, "On Understanding Types, Data Abstraction, and Polymorphism". http://luca

This point has been argued far more eloquently and accurately by Cardelli and Wagner, "On Understanding Types, Data Abstraction, and Polymorphism". http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf See Section 1.3 "Kinds of Polymorphism" (there is a typo in the paper and section 1.4 is mislabeled as "1.3" as well.)