Indeed - he offered no argument that makes any sense - he assumed.  He  mandated out of a sense of power and then had his CDF lackey call what he said, though not issued as infallible, to be forever irrefutable.  It was an incredible bit  of trickery to expand infallibility - that horrible idea that came out of Vatican I and should never have happened even in the limited way it was supposedly declared.  Well, JPII and Ratzinger/BXVI have really made a mess for the church to try to live on in an ever evolving world and universe.