If what she says is true? If a mystic's mana naturally aligns with their owned element then logically, it should be impossible for a mystic to learn an element outside of their inherent category.
That contradicts everything Aldrich has ever known about mana.
"Apologies, Professor, but I can't believe that to be true."
"Of course, you don't. There's no need to apologize," Professor Mariana replied smoothly. "I'd be more surprised if you accepted it without question."
Accepting her theory on a whim would mean disregarding everything he had been taught, everything he understood about mana and its control.
Mana, as he knew it, was flexible, adaptable. That very nature was what allowed mystics with a owned fire affinity to eventually master water-based techniques. If mana were as rigid as her theory implied, then cross-element learning would be impossible.