An introductory textbook on modal logic the logic of necessity and possibility.
From the Back Cover
"Modal Logic" is a textbook on modal logic, intended for readers already acquainted with the elements of formal logic. Brian F. Chellas provides a systematic introduction to the principal ideas and results in contemporary treatments of modality, including theorems on completeness and decidability. Illustrative chapters focus on deontic logic and conditionality. There are nearly 500 exercises.
This has been a rapidly expanding branch of logic over the last ten years, and familiarity with the subject is now regarded as a necessary part of every philosopher's technical equipment. Chellas here offers an up-to-date and reliable guide of the kind that is very badly needed: his exposition is distinguished by its clarity, economy and elegance, and the book is expected to supplant existing textbooks in the field.