In his 2003 book "Terror and Liberalism", Paul Berman argues that distinct cultural boundaries do not exist in the present day. He argues there is no "Islamic civilization" nor a "Western civilization", and that the evidence for a civilization clash is not convincing, especially when considering relationships such as that between the United States and Saudi Arabia. In addition, he cites the fact that many Islamic extremists spent a significant amount of time living or studying in the Western world. According to Berman, conflict arises because of philosophical beliefs various groups share (or do not share), regardless of cultural or religious identity.