Though the Romans added little to the theory of the fundamental problem involved, their history proved rich in practical experience. There were times, in parts of the Roman Empire, when personal slavery either did not exist or was limited and insignificant in extent. But the institution grew with Roman wars and conquests. In rural districts, slave labor displaced free labor, and in the cities servants multiplied with the concentration of wealth. The size and character of the slave population eventually became a perpetual menace to the State. Insurrections proved formidable, and every slave came to be looked upon as an enemy to the public. It is generally conceded that the extension of slavery was a primary cause of the decline and fall of Rome. In the American controversy, therefore, the lesson to be drawn from Roman experience was utilized to support the cause of free labor. After the Middle Ages, in which slavery under the modified form of feudalism ran its course, there was a reversion to the ancient classical controversy. The issue became clearly defined in the hands of the English and French philosophers of the seventeenth and eighteenth centuries. In place of the time-honored doctrine that the masses of mankind are by nature subject to the few who are born to rule, the contradictory dogma that all men are by nature free and equal was clearly enunciated. According to this later view, it is of the very nature of spirit, or personality, to be free. All men are endowed with personal qualities of will and choice and a conscious sense of right and wrong. To subject these native faculties to an alien force is to make war upon human nature. Slavery and despotism are, therefore, in their nature but a species of warfare. They involve the forcing of men to act in violation of their true selves. The older doctrine makes government a matter of force. The strong command the weak, or the rich exercise lordship over the poor. The new doctrine makes of government an achievement of adult citizens who agree among themselves as to what is fit and proper for the good of the State and who freely observe the rules adopted and apply force only to the abnormal, the delinquent, and the defective. Between the upholders of these contradictory views of human nature there always has been and there always must be perpetual warfare. Their difference is such as to admit of no compromise; no middle ground is possible. The conflict is indeed irresistible. The chief interest in the American crusade against slavery arises from its relation to this general world conflict between liberty and despotism.