The Book of Isaiah (, "Sefer Y'sha'yah") is a book of the Bible thought to be written by the Prophet Isaiah, who lived in the second half of the 8th century BC. The Book of Isaiah contains many prophecies about ancient Israel, its people, and its enemies. Tradition says that all 66 chapters of the book were written by Isaiah, but other theories say that some chapters were written by one or more other people. This is mainly because they were about a later period and the text doesn't mention Isaiah after Chapter 39.