“Arai's book represents an important contribution to the field of ordinal analysis, and is recommended to any mathematician interested in the classification of the proof-theoretic strength of mathematical theories. … the book is well written, well organized, and concise … . Arai is to be commended for this significant resource for anyone who wishes to study ordinal analysis, and gain a better understanding of the proof-theoretic strength of theories.” (Matthew Jura, Mathematical Reviews, December, 2023)