This book is only partially new. It is an expansion of Logic and Epistemology, which was first printed in 1930 and has since been reprinted or revised almost each year. The expansion consists in the addition of an Introduction to Philosophy and of Ontology. It had been my original intention to add both parts but other more urgent work stood in the way until w. As in the preface to the former work, I wish to stress a few considerations that have guided me in the composition of this. Some are more general, others are applicable to one or the other of the branches of philosophy here offered. 1. Brevity, plain and direct presentation, close-knit reasoning and logical coherence--these are the aims which have been kept in view throughout. 2. The scholastic system and method were chosen for presentation. Some 30 years of teaching philosophy and theology have convinced me of its pedagogical worth. There is other philosophical system that can rival the scholastic in common sense and definiteness. There is other philosophical method that even approaches the scholastic in sound reasoning and logical force. To the beginner, who is t yet capable of grappling with superficial systems and subtle methods, such a philosophy is indispensable. 3. The Introduction to Philosophy differs considerably from those current today. It is t a summary of philosophical problems, which the beginner cant digest and which may engender in him a spirit of skepticism. For a like reason I have t given an outline of the history of philosophy with the same problems arranged by periods. Instead, I confined myself to a few important data on the principal philosophers of past ages, and I tried to sketch the intellectual equipment with which the student is supposed to begin philosophy. 4. In Logic, the simple syllogism has been made the unifying theme as well as the ultimate aim. On the other hand, the sentence or proposition has been made the starting point of all explanations; the reason for this is the well kwn axiom that all truths are expressed by us in sentence form. 5. Nor-let it be said even more emphatically-is Epistemology a branch of psychology. Epistemology investigates the final cause of our cognitive faculties, psychology the other three causes (formal, material, efficient). Again, the four causes are present in all cognition, but Epistemology restricts itself to the purpose of cognition. 6. In Ontology, it has been my endeavor to eliminate the Latin flavor and to substitute for it readable and plain English. Special attention has been given to the bibliography and the alphabetical index. The former lists practically all English Catholic books on the subject, including both earliest and latest publications. The latter has been made very detailed so as to insure the utmost of usefulness.