Principles may well be regarded as the main part of philosophy. They are among the major discoveries of philosophy, condensing in themselves much philosophical inquiry and insight. They are the starting point of much philosophical discussion. They are the base for exposition, for proof, and for criticism. They serve the student and the reader of philosophy much as legal maxims serve jurists and as proverbs serve the people. They are for scholastic philosophers the household truth of their tradition. This book includes not only all principles of scholastic philosophy but also exercises to apply the principles to several occasions. The book is useful for all students and professionals in philosophy.