The title Father of the Church, which has its origin in the name of .. Father" given to bishops 1 as early as the second century, was commonly used in the fifth century to designate the old ecclesiastical writers - ordinarily bishops - who died in the faith and in communion with the Church. According to modern theologians, the title applies only to those writers who have the four following qualifications: orthodoxy of doctrine, holiness of life, ecclesiastical satlctiotJ, and antiquity. Practically, however, it is given to many others who do not possess the first three requisites. Nobody, indeed, would dream of eliminating from the list of the "Fathers" such men as Tertullian, Origen, Eusebius of Caesarea, Faustus of Riez, etc. Errors have been laid to their charge, but these mar their works without making them more dangerous than useful; whilst they are wrong on a few points, there is in them much that is good. At all events, they eminently deserve the title of Ecclesiastical Writers. However comprehensive may be the name "Fathers of the Church," Patrology is the study of the life and works of the men designated by that name. As a science, then, it is part of the History of Ancient Christian Literature, since it excludes from the field of its labors both the canonical writings of the New Testament and all writings that are strictly and entirely heretical. On this latter point, however, most authors exercise a certain tolerance. As a knowledge of heretical works is very often useful, nay even necessary, for understanding the refutations written by the Fathers, most Patrologies do not hesitate to mention and describe at least the principal ones. We will follow this method: not mentioning the New Testament writings, but describing, in part at least, and very briefly, the heterodox writings best known in the early centuries. The question may be raised here: Is Patrology to comprise not only the history of the life and works of the Fathers, but also a summary of their doctrine: that is, must Patrolo supply the elements of a Patristic Theology? TheoretIcally, yes; but in practice nothing could be more difficult. A Patrology which would attempt to give even a very condensed summary of the teaching of each and every Father would have to be very lengthy and full of repetitions. If, on the other hand, such a work simply pointed out teachings not original and instead limited itsel f to what is proper and personal in each, it would give a false - because incomplete - impression of each author's doctrine. For this reason we think it better to draw a line of strict demarcation between Patrology and Patristic Science and leave the teaching of the Fathers to the History of Dogma. The two sciences cannot but gain by being studied separately. The most Patrology can do is to indicate, in the case of some of the Fathers, the points of doctrine they have best illustrated.