Pope Paul VI
Sacerdotalis Caelibatus §61
Sacerdotalis Caelibatus: On the Celibacy of the Priest
61 The Second Vatican Council has already indicated wise criteria and guidelines to this end. They are in conformity with the progress of psychology and pedagogy, as well as with the changed conditions of mankind and of contemporary society. (115) It is Our wish that appropriate instructions be drawn up with the help of truly qualified men, treating with all necessary detail the theme of chastity. They should be sent out as soon as possible to provide competent and timely assistance to those who have the great responsibility within the Church of preparing future priests.
Source: Sacerdotalis Caelibatus (Vatican.va)