Quoth the raven (XLIX):

(...)
Urza smiled. "I must go away," he repeated.
Tawnos nodded slowly, and the man with human/crystalline eyes stood. "You have long been a student," said Urza. "Now go be a teacher." As he spoke, Urza began to fade from view. Slowly the color drained from him, leaving only outlines; then they too faded. "Teach them of our triumphs and our mistakes,"said a distant voice. "And tell Kayla to remember me not…"
"... As you were, but as you tried to be," finished Tawnos, but he was speaking to empty space. Urza had passed from the world into greater worlds that only his crystalline eyes could see. (...)

Jeff Grubb, The Brothers War (Artifacts Cycle, book I)

Now I finally see what this truly means.