consider so long as they are subordinated to the primary end and so long as the intrinsic nature of the act is preserved.}} John and Sheila Kippley from the Couple to Couple League say that the statement of Pope Pius XI not only permitted sex between married couples during pregnancy and menopause, but also during the infertile times of the menstrual cycle. Raymond J. Devettere says that the statement is a permit to undertake intercourse during the infertile times when there is "a good reason for it". The mathematical formula for the rhythm method had been formalized in 1930, and in 1932 a Catholic physician published a book titled The Rhythm of Sterility and Fertility in Women promoting the method to Catholics. The 1930s also saw the first U.S. Rhythm Clinic (founded by John Rock) to teach the method to Catholic couples. However, use of the rhythm method in certain circumstances was not formally accepted until 1951, in two speeches by Pope Pius XII. This method has come to be known as "natural family planning." Some studies suggest that couples who practice NFP enjoy an abundance of healthy advantages, the most notable of which might be a reduced divorce rate amongst couples who use NFP. The Catholic Church continues to uphold the practice of natural family planning since it maintains the unitive aspect of the sexual union while still bolstering an openness to procreation. Current view The Catholic position on contraception was formally explained and expressed by Pope Paul VI's Humanae vitae in 1968. Artificial contraception is considered intrinsically evil, but methods of natural family planning may be used, as they do not usurp the natural way of conception. In justification of this position, Pope Paul VI said: Responsible men can become more deeply convinced of the truth of the doctrine laid down by the Church on this issue if they reflect on the consequences of methods and plans for artificial birth control. Let them first consider how easily this course of action could open wide the way for marital infidelity and a general lowering of moral standards. Not much experience is needed to be fully aware of human weakness and to understand that human beings—and especially the young, who are so exposed to temptation—need incentives to keep the moral law, and it is an evil thing to make it easy for them to break that law. Another effect that