Through me the way into the grieving city, Through me the way into eternal sorrow, Through me the way among the lost people. Justice moved my high ma… - Dante Alighieri

This seeming incongruity has long troubled me. I owe Professor MacAllister a glad thanks for what is certainly the essential clarification. The whole Purgatorio, he points out, is built upon the structure of a Mass. The Mass moreover is happening not on the mountain but in church with Dante devoutly following its well-known steps. I have not yet had time to digest Professor MacAllister’s suggestion, but it strikes me immediately as a true insight and promises another illuminating way of reading the Purgatorio.

It is necessity and not pleasure that compels us.
[Italian: Necessita c'induce, e non diletto.]

Wherefore it would appear that this number was thus allied unto her for the purpose of signifying that, at her birth, all these nine heavens were at perfect unity with each other as to their influence. This is one reason that may be brought: but more narrowly considering, and according to the infallible truth, this number was her own self: that is to say, by similitude. As thus. The number three is the root of the number nine; seeing that without the interposition of any other number, being multiplied merely by itself, it produceth nine, as we manifestly perceive that three times three are nine. Thus, three being of itself the efficient of nine, and the Great Efficient of Miracles being of Himself Three Persons (to wit: the Father, the Son, and the Holy Spirit), which, being Three, are also One: — this lady was accompanied by the number nine to the end that men might clearly perceive her to be a nine, that is, a miracle, whose only root is the Holy Trinity. It may be that a more subtile person would find for this thing a reason of greater subtilty: but such is the reason that I find, and that liketh me best.