The present work is prompted by the prize question posed by the theological faculty of the University of Munich for the year 1892/93: "The teaching of St. Thomas Aquinas on property should be compared with the corresponding views of modern socialism." In the discussion, the conflicting parties initially have their say, and only as necessary and appropriate does the remaining pro and contra literature, as well as other related friendly and hostile directions, come into play. However, the author gratefully notes that he has gained much inspiration from that literature. The author believes he has delved deeply enough into both sides (Thomas - Socialism) in writing and spirit to provide a fundamentally sound systematic compilation. This book is about the doctrine of property according to Thomas Aquinas and modern socialism.