Good afternoon in the other side of the globe!
It is a theological speculation elevated to dogma, a "theologumenon" (theological opinion), based on a reasoning that turns a hypothesis into truth: "If [God] could do it... he did it" The foundation of this argument would be the "exaggeration" of God's grace, which would always be super-abundant, and would always exhaust all possibilities. His author has been the "Doctor Subtilis", Duns Scotus, a medieval theologian from Scholasticism - a very peculiar philosophical-theological stream, as you may know.
The millennial tradition of the undivided Western-Eastern Church (from the first millennium - and even from a big part of the western second millenial tradition) recognizes this possibility as ... mere possibility, mere theological reflection or mere devotional-affective truth of free adherence. In fact, St. Gregory Palamas, the bastion of (Eastern) Orthodoxy, freely defended it, and in a very particular way.
But the assumptions of the idea of the Immaculate Conception seem to carry complicated biases in relation to anthropology and sin (to the themes of freedom and humanity). And the fact that it has been "dogmatized" brings very serious consequences for ecumenical dialogue and any chance of re-union of the Churches.
You can read more about the origin of the dogma in Theology Manuals or Dictionaries, in a more western or more ecumenical view, depending on the book.
Last edited by Philippe Gebara; 02/13/18 10:42 PM.