Until the middle of the 18th century, slavery was practiced with little challenge anywhere in the world. For centuries philosophers as varied as Aristotle, Thomas Aquinas, and John Locke accepted slavery as part of a proper social system. However, across Europe through the last part of the 18th century there were intellectual antislavery arguments based on Enlightenment thought, as well as moral arguments (notably among Quakers, in Great Britain and the United States) which questioned the legitimacy of slavery. Only in the American Revolutionary War era did slavery first become a significant social issue in North America. In the North, beginning during the Revolution and continuing through the first decade of the next century, state by state emancipation was achieved by legislation or lawsuit although in the larger slaveholding states such as New York and Pennsylvania emancipation was gradual. By 1810, 75% of Northern slaves had been freed and virtually all were freed within the next generation.