Josquin begins with a Dorian cadence above the “profound” low D:

He then cadences on the Aeolian A, a fifth above Dorian D, though he only includes its fundamental (A) and fifth (E), not the third (F):

As he gradually introduces the missing F, he then leads it down a semitone to E and uses that as the third degree of C (Ionian), on which he then cadences:

This leads to a subsequent cadence on G, the Ionian dominant (that is, the fifth above its final):

But G is also the third degree of E; this reinterpretation of G enables the ensuing Ionian (C) harmony to be the pivot between Dorian harmonies and Phrygian. Josquin underlines the modal distance so far traversed by ending the motet’s first part on a full Aeolian (A) cadence, now bringing forward in the highest voice its third (C), which also alludes to the important Ionian (C) arrivals that had prepared it: