Principle #15 states that there can be seeming contradictions between magisterial statements. As an addendum to this, we have principle #16:
A magisterial statement cannot be judged false, erroneous, heretical, etc., simply because it appears to contradict a former magisterial statement.
This principle follows from principle #3, that the Magisterium has everything necessary to accomplish its purpose. For if principle #16 were false, every doctrine would have to be decided on the basis of private judgment, so that the Magisterium would be rendered pointless. This can be illustrated by an example. It has occasionally been argued that Pius XII taught heresy when he affirmed the moral liceity of natural family planning, on the grounds that Pius XI condemned the practice. But if this is possible, how do we know that it was not Pius XI who taught heresy, and that the teaching of Pius XII was not the true one? Someone might equally well argue that we know that Pius XI was a heretic because he contradicts the teaching of Pius XII.
There are two possible answers to this question. One might say, first, that it is in virtue of the content of the teaching that we know it was Pius XII, not Pius XI, who was the heretic. But in order to make such a judgment, we must first know which teaching is true, and therefore cannot depend on the Magisterium to educate us on this matter. According to this account, the Magisterium is useless.
On the other hand, one might say that it is because Pius XI’s teaching temporally preceded that of Pius XII that we know the former’s is the authoritative teaching. This reply has two problems. First, it violates principle #10, which states that later magisterial documents have greater weight than earlier ones, all other things being equal. This ultimately results in the false doctrine of Sola Scriptura, since the only way we can be sure that a later teaching is not heretical is if it appears to us to agree with the earliest teaching of all; this too renders the Magisterium pointless. Second, if Pius XII was able to teach heresy, then so was Pius XI, and we are back to the initial problem of having to depend on our private judgment. This will be true unless one argues either that the temporal precedence of Pius XI causes his teaching to be true, or that a pope’s teaching is infallible only if he is the first pope to speak on the subject. Both claims are manifestly absurd.
Hence, the appropriate response to apparent contradictions in magisterial teaching is, first, suspension of judgment, and second, careful consideration of whether one’s interpretation of the earlier teaching might in fact be erroneous.