In the last half of the nineteenth century there appeared in the United States a group of political theorists differing from the earlier thinkers in respect to method and upon many important doctrines of political science. Much of the credit for the establishment of the new school belongs to Francis Lieber, a German scientist who came to this country in 1827, and, as an educator and author, left a deep impress on the political thought of America. The individualistic ideas of the "natural right" school of political theory, indorsed in the Revolution, are discredited and repudiated. The notion that political society and government are based upon a contract between independent individuals and that such a contract is the sole source of political obligation, is regarded as no longer tenable. In the refusal to accept the contract theory as the basis for government, practically all the political scientists of note agree.