  The author wishes to express his deep gratitude to Phillip Bricker and Max Cresswell for extensive comments on several drafts of this entry and for numerous illuminating discussions of its content and related topics. The entry is vastly better for their generous input. Errors and other infelicities that remain are of course the sole responsibility of the author. A great deal of this entry was written with the support of the Alexander von Humboldt Foundation while the author was a Visiting Fellow at the Munich Center for Mathematical Philosophy in 2011–12.  Thanks are due to the Center's director, Professor Hannes Leitgeb, for making the author's stay at this remarkable venue possible.  Finally, the author would like to express his thanks to the SEP Editors for their extraordinary patience in dealing with the very tardy author of a badly-needed entry.