Open question: what is the significance of papal encyclicals? To what extent are they binding teaching, especially where they contradict each other? Are we to look at them as loquacious pastoral letters concerning whatever happens to be on the pope's mind?
The difficulty is that the encyclical replaced the papal bull around the time of Gregory XVI. Previously, popes would write "bulls" to the bishops and priests of the Church to legislate with regard to a particular problem or a binding teaching to clarify a matter of dispute. Then came the Italian unification movement and the popes began to write about more general issues. The culture of the time and the rather dull theological outlook were not conducive to considering the contents of encyclicals, only receiving them. Indeed, when traditionalists speak of "traditional Roman Catholic theology" they mean the encyclicals of Gregory XVI, Pius IX, most of Leo XIII, Pius X, Pius XI, and Pacelli. These letters were to be taken as typical Catholic teaching on all matters much as how neo-conservatives take John Paul II and Francsis' encyclicals to be gospel, as well as Humane Vitae (which we must admit is not a teaching, but a re-iteration of a long standing teaching). Then Cardinal Burke caused a small firestorm by suggesting Francis' first unique encyclical was not "Magisterium."
Another problem is that, regarding papal bulls, prior to Vatican I, one is not entirely clear as to how binding bulls were. Some were purely legislative. Some, like Benedictus Deus, were clearly didactic and binding. And others, like Unam Sanctam, were shameless political posturing. We can dangerously read into formulae like "we define" and "we declare" a level of authority generally used in one era, but bound to signify invocations of infallibility in a later era. Is the fine for mis-printing the Roman Missal stated in Quo primum tempore a dogma of the faith?
In all the centralization and bureaucracy, no one seems very keen on separating the essentials of teaching from what is pastoral care and what is just irrelevant. Thoughts?