The pope is infallible when, as head of the bishops of the church, he requires the faithful to believe a matter pertaining to faith or morals. This is the standard definition of infallibility. It goes no further. Should the pope stick his head out of doors and remark that it will be a nice day, he is just as liable to be snowed on as the next man.

In practice, this means that the church debates a subject at great length and when the bishops and laity are in agreement, the pope makes an infallible pronouncement. The bishops' discussions before the announcement can take a long time; the doctrine of the immaculate conception was in debate for at least 1,300 years before being made official....