This is an open access title distributed under the terms of the CC BY-NC 4.0 license, which permits any non-commercial use, distribution, and reproduction in any medium, provided no alterations are made and the original author(s) and source are credited.Further information and the complete license text can be found at https://creativecommons.org/licenses/by-nc/4.0/The terms of the CC license apply only to the original material. The use of material from other sources (indicated by a reference) such as diagrams, illustrations, photos and text samples may require further permission from the respective copyright holder.An electronic version of this book is freely available, thanks to the support of libraries working with Knowledge Unlatched. More information about the initiative can be found at www.knowledgeunlatched.org.Cover illustration: Honorific statue of Lucius Antonius Claudius Dometeinus Diogenes. Aphrodisias, ca. ad 200. Picture licensed under the Attribution-ShareAlike license (cc by-sa 2.0). Photo by Carole Raddato. Reproduced by kind permission of the photographer (https://followinghadrian.com).