  Aczel’s book was also immediately influential for people working on semantic questions in theoretical computer science.  This was not so much because it raised questions about set theory, but rather because it showed the value of using the categorical notion of a coalgebra.  The main use in the book is to organize certain concepts into an elegant subject.  But it quickly became apparent that this notion of coalgebra could be studied on its own, that themes from the book had a field of application much wider than pure set theory.