The literature relating to the work of Piaget is large and still growing. Some of it is Piagetian; some of it is critical. Most of this has been directed towards his experimental methodology and the conclusions drawn from it. The justification for the present contribution lies in what the authors believe to be the special embodiment in Piagetian thought of a central theme of our time. This theme is that the only possibility of truth lies in measurability and that knowledge is not recognisable unless it satisfies this criterion. This work is concentrated in the first instance on Piaget's claims that mental structures are exclusively logical mathematical in form, especially since this part of his work has received least attention. This book was first published in 1985.