books.google.com - In this volume, Lambek and Scott reconcile two different viewpoints of the foundations of mathematics, namely mathematical logic and category theory. In Part I, they show that typed lambda-calculi, a formulation of higher-order logic, and cartesian closed categories, are essentially the same. Part II...http://books.google.com/books/about/Introduction_to_Higher_Order_Categorical.html?id=6PY_emBeGjUC&q=natural+transformation&utm_source=gb-gplus-shareIntroduction to Higher-Order Categorical Logic