“This book contains very important developments in Piaget’s theory, as in his treatment of knowledge he concentrated more and more on psychological-procedural processes of equilibration and less on logical-structural models. He considered the ‘opining up of infinite possibilities’ the central problem of any viable theory of knowledge in that it created a dialectical tension with the (almost contradictory) need for closure, proper to any biological system. Possibility and Necessity is an important empirical contribution to clarify equilibration, a concept which is perhaps the most important in his theory, and which he worked on and revised for fifty years.”