 Complete the following phrase:  Encyclicals indicate high Papal priority for an issue at a given time. Only pontiffs define when, and under which circumstances encyclicals should be issued. They may choose to issue an apostolic constitution, bull, encyclical, apostolic letter or give a papal speech. Pontiffs differ on the use of encyclicals. On the issue of birth control and contraception, for example, Pope Pius XI issued the encyclical Casti connubii, while Pope Pius XII spoke to midwives and the medical profession when he clarified his position on the issue. Pope Paul VI published an encyclical Humanae vitae on the same topic. On matters of war and peace, Pope Pius XII issued ten encyclicals, most of them after 1945, three of them Datis nuperrime, Sertum laetitiae, and Luctuosissimi eventus protesting the Soviet invasion and crackdown of the 