The Vinaya Pitaka, the first division of the Tipitaka, is the textual framework upon which the monastic community (Sangha) is built. It includes not only the rules governing the life of every Theravada bhikkhu (monk) and bhikkhuni (nun), but also a host of procedures and conventions of etiquette that support harmonious relations, both among the monastics themselves, and between the monastics and their lay supporters, upon whom they depend for all their material needs.
When the Buddha first established the Sangha, the community initially lived in harmony without any codified rules of conduct. As the Sangha gradually grew in number and evolved into a more complex society, occasions inevitably arose when a member would act in an unskillful way. Whenever one of these cases was brought to the Buddha's attention, he would lay down a rule establishing a suitable punishment for the offense, as a deterrent to future misconduct. The Buddha's standard reprimand was itself a powerful corrective:
It is not fit, foolish man, it is not becoming, it is not proper, it is unworthy of a recluse, it is not lawful, it ought not to be done. How could you, foolish man, having gone forth under this Dhamma and Discipline which are well-taught, [commit such and such offense]?... It is not, foolish man, for the benefit of un-believers, nor for the increase in the number of believers, but, foolish man, it is to the detriment of both unbelievers and believers, and it causes wavering in some.
— The Book of the Discipline, Part I, by I.B. Horner (London: Pali Text Society, 1982), pp. 36-37.
The monastic tradition and the rules upon which it is built are sometimes naïvely criticized — particularly here in the West — as irrelevant to the "modern" practice of Buddhism. Some see the Vinaya as a throwback to an archaic patriarchy, based on a hodge-podge of ancient rules and customs — quaint cultural relics that only obscure the essence of "true" Buddhist practice. This misguided view overlooks one crucial fact: it is thanks to the unbroken lineage of monastics who have consistently upheld and protected the rules of the Vinaya for almost 2,600 years that we find ourselves today with the luxury of receiving the priceless teachings of Dhamma. Were it not for the Vinaya, and for those who continue to keep it alive to this day, there would be no Buddhism.
It helps to keep in mind that the name the Buddha gave to the spiritual path he taught was "Dhamma-vinaya" — the Doctrine (Dhamma) and Discipline (Vinaya) — suggesting an integrated body of wisdom and ethical training. The Vinaya is thus an indispensable facet and foundation of all the Buddha's teachings, inseparable from the Dhamma, and worthy of study by all followers — lay and ordained, alike. Lay practitioners will find in the Vinaya Pitaka many valuable lessons concerning human nature, guidance on how to establish and maintain a harmonious community or organization, and many profound teachings of the Dhamma itself. But its greatest value, perhaps, lies in its power to inspire the layperson to consider the extraordinary possibilities presented by a life of true renunciation, a life lived fully in tune with the Dhamma.