Commentary Purg I 40-45

Cato's initial rigid and probing moral attitude may seem to indicate that he does not immediately understand the very grace that has brought him here.  He reasons that Dante and Virgil, not arriving at his shores in the 'normal' way (disembarking from the angel's ship that we shall see in the next canto), may have snuck into this holy land.  He intuits that they have come up from the stream (the eventual course of Lethe?) that descends into hell (see [Inf XXXIV 129-132]) and is eager to know how they could have done so without a very special grace indeed.  Nonetheless, in a manner totally unlike that encountered in the demons of Inferno, he at once allows for the possibility of grace.  His second tercet immediately reveals what a different place we have now reached, one in which doubt and possibility exist even in the minds of its sternest keepers.