A plan of life after the spirit of the Gospel, imposed on members of religious orders to help them work in common for the special objects of their institute.
The first rules were plans of perfect life, with details differing according to persons, times, and places, but based on the Gospel, and found in the Acts of the Apostles.
As orders began to approach their modern form, and new ones were founded having their own special objects, each institute had its own rule.
In the beginning the rule was simply a short code of asceticism, with such directions as were necessary for the organization of common life; and in the orders properly so called, there were added to this code the regulations required by the special objects of each institute.
The four great rules are those of Saint Basil, governing most monks of the Greek Rite; Saint Benedict, the principal rule of the Western monks; Saint Augustine, common to the Canons Regular, the Hermits of Saint Augustine, and many others; and Saint Francis of Assisi, observed by the three branches of his first order, the second order, and many congregations of tertiaries.
The Carmelites, the Minims, the Society of Jesus, the Passionists, and the Redemptorists all have their own constitutions only.
New Catholic Dictionary