The first rule in the Pāṭimokkha opens with the statement that it — and, by extension, every other rule in the Pāṭimokkha — applies to all bhikkhus who have not disrobed by renouncing the training and returning to the lay life. Thus the Vibhaṅga begins its explanations by discussing what does and does not count as a valid act of disrobing. Because this is, in effect, the escape clause for all the rules, I am discussing it first as a separate chapter, for if a bhikkhu disrobes in an invalid manner, he still counts as a bhikkhu and is subject to the rules whether he realizes it or not. If he then were to break any of the pārājika rules, he would be disqualified from ever becoming a bhikkhu again in this lifetime.

The Dhamma and Vinaya impinge in such detail on so many areas of one's life that no new bhikkhu can be expected to master them in a short time. For this reason, the Buddha arranged for a period of apprenticeship — called nissaya, or dependence — in which every newly ordained bhikkhu must train under the guidance of an experienced bhikkhu for at least five years before he can be considered competent to look after himself.

The meaning of the term pāṭimokkha is a matter of conjecture. According to the Mahāvagga it means "the beginning, the head (or entrance — mukha), the foremost (pamukha) of skillful qualities" (Mv.II.3.4). The term serves as the name not only of the basic code of training rules, but also of a sermon in which the Buddha enumerated the basic principles common to the teachings of all Buddhas: "The non-doing of all evil, the performance of what is skillful, and the purification of one's mind: This is the Buddhas' message" (Dhp. 183). Thus whatever the etymology of the term pāṭimokkha, it denotes a set of principles basic to the practice of the religion.

Introduction to the Vinaya incl. an explaining of the sources and it's values.