Political scientist Samuel Huntington has argued that the defining characteristic of the 21st century will be a clash of civilizations. According to Huntington, conflicts between civilizations will supplant the conflicts between nation-states and ideologies that characterized the 19th and 20th centuries. These views have been strongly challenged by others like Edward Said, Muhammed Asadi and Amartya Sen. Ronald Inglehart and Pippa Norris have argued that the "true clash of civilizations" between the Muslim world and the West is caused by the Muslim rejection of the West's more liberal sexual values, rather than a difference in political ideology, although they note that this lack of tolerance is likely to lead to an eventual rejection of (true) democracy. In "Identity and Violence" Sen questions if people should be divided along the lines of a supposed "civilization", defined by religion and culture only. He argues that this ignores the many others identities that make up people and leads to a focus on differences.