The question of the nature of the purgatorial fire was raised at the Council of Florence, the Greeks insisting upon a symbolic understanding. The council wisely avoided settling this question.
But you arrived on time, as expected :-)
Here is why +BXVI is THE POPE of the past 1000+ years:
“Some recent theologians are of the opinion that the fire which both burns and saves is Christ himself, the Judge and Saviour. The encounter with him is the decisive act of judgement. Before his gaze all falsehood melts away. This encounter with him, as it burns us, transforms and frees us, allowing us to become truly ourselves. All that we build during our lives can prove to be mere straw, pure bluster, and it collapses. Yet in the pain of this encounter, when the impurity and sickness of our lives become evident to us, there lies salvation. His gaze, the touch of his heart heals us through an undeniably painful transformation “as through fire”. But it is a blessed pain, in which the holy power of his love sears through us like a flame, enabling us to become totally ourselves and thus totally of God. In this way the inter-relation between justice and grace also becomes clear: the way we live our lives is not immaterial, but our defilement does not stain us for ever if we have at least continued to reach out towards Christ, towards truth and towards love. Indeed, it has already been burned away through Christ’s Passion. At the moment of judgement we experience and we absorb the overwhelming power of his love over all the evil in the world and in ourselves. The pain of love becomes our salvation and our joy.”
and
“Purgatory is not, as Tertullian thought, some kind of supra-worldly concentration camp where man is forced to undergo punishment in a more or less arbitrary fashion. Rather is it the inwardly necessary process of transformation in which a person becomes capable of Christ, capable of God and thus capable of unity with the whole communion of saints. Simply to look at people with any degree of realism at all is to grasp the necessity of such a process. It does not replace grace by works, but allows the former to achieve its full victory precisely as grace. What actually saves is the full assent of faith. But in most of us, that basic option is buried under a great deal of wood, hay and straw. Only with difficulty can it peer out from behind the latticework of an egoism we are powerless to pull down with our own hands. Man is the recipient of the divine mercy, yet this does not exonerate him from the need to be transformed. Encounter with the Lord is this transformation. It is the fire that burns away our dross and re-forms us to be vessels of eternal joy.”
What further definitions do we need than this? This is within the consensus patrum! This is Christianity!