Here is a difference in theological belief where there seems little chance of agreement. The grounds for the Church's position are Scriptural (Old Testament), the teachings of the fathers and doctors of the early Church, the unbroken tradition of nineteen centuries, the decisions of the highest ecclesiastical authority and the natural law. The latter plays a prominent role in Roman Catholic theology and is considered decisive, entirely apart from Scripture, in determining the ethical character of birth-prevention methods.

The Roman Catholic natural-law tradition regards as self-evident that the primary objective purpose of the conjugal act is procreation and that the fostering of the mutual love of the spouses is the secondary and subjective end. This conclusion is based on two propositions: that man by the use of his reason can ascertain God's purpose in the universe and that God makes known His purpose by certain ``given'' physical arrangements.

Thus, man can readily deduce that the primary objective end of the conjugal act is procreation, the propagation of the race. Moreover, man may not supplant or frustrate the physical arrangements established by God, who through the law of rhythm has provided a natural method for the control of conception. Believing that God is the Author of this law and of all laws of nature, Roman Catholics believe that they are obliged to obey those laws, not frustrate or mock them.