The aim of seminaries managed by the Congregation of the Mission had influence on the scope and duration of studies and the instructions of Vincent a Paulo denoted the formation direction of future priests, who were expected to posses high intellectual and spiritual qualifications. To achieve this aim an adequate set of works by the most renowned theologians and scholars was required. According to the rules in effect in the seminaries managed by the Congregation of the Mission, the obligatory reading was meant to constitute a full intellectual and spiritual harmony. All this demanded an adequate and comprehensive knowledge of theology and law. The choice of textbooks and additional reading was determined by the criterion laid down during the general conventions. Books that raised doctrinal objections and did not have the Holy See approval were to be avoided above all. Those which were most suitable for seminarists as far as their volume and the manner of the discourse went as well as those easily accessible for the alumni were recommended. Apart from works from the inventory of the Library, professors of the Płock semi-nary based their lectures on their own textbooks, various excerpts assuredly prepared on the basis of many other texts constituting thematic syntheses.