In terms of outward form, magic undoubtedly had many similarities to psionics.
But Guilliman was, of course, well aware that the magic He Shenyan spoke of and the psionics he knew so well were two different things.
Psionics was a power that had yet to be fully developed. Even now, humanity's understanding of it went no further than dividing it into five schools. That classification had been made by the Thousand Sons before the Great Heresy, and a full ten thousand years had passed since then.
What could psykers do?
Well, many things. Vague prophecies, mind reading, environmental perception, even foreseeing an opponent's next thought in battle. Teleportation, telekinesis, manipulation of time, elemental empowerment—a whole range of methods beyond the scope of science could all be classified as psionics.
However, powerful psykers were very rare—in the end, because this ability originated from the Warp. When psykers used psionics, their souls were also exposed to the gaze of the Demons of the Warp.
Dangerous. Uncontrollable. The reason humanity still used it was because there was no other way.
They had to survive. For that purpose, let alone risks—even morality and humanity could be cast aside when necessary.
"...Magic." Guilliman spoke, his voice hoarse. "What exactly is it?"
He Shenyan snapped his fingers and conjured an armchair. Same as ever: velvet, pale violet lining, square armrests that let him tap out a crisp sound with his fingers.
"I appreciate your attitude, Guilliman. Some people believe magic should not be defined, that it is a complex and idealistic power. But in truth, magic can be understood, can be categorized, and can even be fully mastered."
The Mage smiled.
"I used to say that magic was a miracle. Yes, part of it is indeed miracle. But now I've discovered... not all of it is."