Feliks Gross writes:
Usually after a year and a day spent in the city fugitive serfs gained freedom. "The air of the city makes you free," was a German maxim ("Dies Stadtluft macht frei"). The statute of limination secured the rights of a fugitive serve and nullified the rights of the lord. "Birth meant little," continues Pirenne,"whatever might be the mark which it had stigmatized the child in his cradle it vanished in the atmosphere of the city." The city, in fact, was the only place that secured freedom to a fugitive peasant, tired of servitude and oppression. It was the only escape. Once in the city, he was free. The charter of establishment of the commune of Saint Quentin (1151) declared: "It does not matter who he is, nor whence he comes. If he is not a thief, he may life in the commune, and, from the moment he enters the city, no one can raise his hand to him nor to do him violence." 
George McLean and John Kromkowski argue that the proverbial 15th century peasant who claimed that "Stadtluft macht frei" was drawing a line between the public and the private:
Consider the tension revealed here between the qualities perceived in village life and those anticipated in the city. Village represents a smothering community. An homogeneity of tastes, styles and desires is inscribed on each villager's soul by an intrusive familiarity that begins in the cradle. The village represents a life lived with intimate, ubiquitous authorities wherein all is public. City, for our peasant, offers the heterogeneity of anonymity and the possibility of private spaces resistant to the intrusive, public scrutiny found in village life. In the peasant's ideal of the city there is room for private space and authority is formal, not intimate or personal. 
Within this free private space, commerce, art and, later, science and industry thrived. Cities are creative places.
 Feliks Gross, Citizenship and Ethnicity, Greenwood Publishing Group, 1999, p. 57.
 Stephen Scheck, "City and Village," in George McLean and John Kromkowski, Urbanization and Values, Washington DC, 1991, p. 170.