Back when Zhang Yu had trapped Daoist Shao with the Heart Seal, there were certain principles he himself did not know, yet Daoist Shao had been able to learn of them through the mouths of those transformed others.
This should all have been aided by the Heart Seal, stemming from the Great Dao's completion. Yet the specific transformations could not be summed up in a single sentence; they involved even deeper mysteries.
Zhang Yu had formed a conjecture at the time. Later, as he delved deeper into the workings of the Heart Seal, his inference was increasingly confirmed.
The Heart Seal could complete these things because it could take itself as the foundation and reflect another self upon it.
The difference was that everything reflected by the Heart Seal was real; that other self truly existed as well. Yet those who cultivated the Dao of Seeking Completeness slew all their selves and achieved the sole and unique self, so the latter was swiftly overlaid by the former.
But everything that other self experienced might differ from his own experiences. Thus, it could obtain different knowledge and principles. Once all of these were gathered, they could make up for shortcomings in divine abilities, Dao arts, and even cultivation.
Nor could he rule out that the people he questioned were reflections of those encountered by that other self. The exact answer depended on the specific transformation, but it should roughly be so.
No matter how one put it, those who cultivated the Dao of Seeking Completeness severed all external selves, yet the Heart Seal could unfold them anew.
Viewed on a greater scale, this seemed to correspond to Origin Xia slaying the myriad ages and Celestial Xia evolving the myriad ages.
Whether slaying the myriad ages or evolving the myriad ages, neither