The canines had done their best to explain the concept of time travel to him, and the scientists had spoken long on the subject of relativity and so on, but it wasn't until Ikra that he began to fully grasp what it would mean to undo the future that was Aku.
It had seemed so simple at first. Go back in time and defeat Aku before he had a chance to grow in power and cause the world Jack found himself in now. A concept foreign enough to make understanding it feel like a victory, but, in the end, simple. But by the time he found himself traveling through the desert with a mysterious warrior woman, he had spent enough time in the future to realize the complexity of undoing something so momentous so far in the past. What had the young scientist called it? The grandfather paradox. It had taken a few hours of explanation and much rephrasing, but he had eventually grasped the concept. Killing your grandfather before you were born meant you were never born, and so therefore never went back in time to kill your grandfather: a paradox.
And what a grandfather! Making it so that Aku had never risen to power would change so much. Ikra was largely who she was because Aku had kidnapped her father (something else they had in common). But even more than that, her race had come to earth directly because Aku had opened up the earth to other planets. Would Ikra even have been born in a future where Aku's rise to power had been nipped in the bud? How many lives would be changed, or completely undone, because Jack fulfilled his destiny?
It wasn't something he liked to think about much, and anyway when Ikra's true identity had been revealed, it hadn't seemed to matter very much anymore. But on occasion the thought surfaced again, in village or city or wilderness: should he succeed, every single person he met would no longer exist; would never have existed in the first place.
It was another depressing thought on top of a surplus of depressing thoughts.