Penal institutions as a principle method of punishment of criminal activity date to the beginning of the nineteenth century. In that era, people considered that imprisonment was a rational way to implement the classical penal philosophy of proportionality, in that the time served related to the severity of the crime. However, skepticism about the results of the new Philadelphia and Pennsylvania correctional systems in the United States had already emerged toward the end of the nineteenth century. Some argued that this form of isolation, causing mental diseases, was the greatest penological mistake of the century.