What if one pope contradicts another?

First, we must determine whether it is a matter of doctrine or morals. If it is not, popes are free to disagree with each other. If it is, we must determine if one or both of the popes is speaking merely as a private theologian. If one of them was speaking definitively in his capacity as pope, then the presumption of correctness would go to him. We must also consider the possibility that what appears to be a contradiction between past and present popes may not be. Popes can state the same truth in a variety of ways without contradicting each other. There may also be theological development that will give nuances to earlier doctrinal pronouncements.