"Honestly, it was just a thought," Hiccup remarked with half a roll of his shoulders, although his tone is far from dismissal. He breathes out as he considers further. "Anything's possible, right?" he adds decisively, "I mean, if it doesn't seem like something outwardly physical or mental... wouldn't that also make sense? They make a big deal of souls around here, as if they have some other meaning."

As if, as they are told to believe, that it's just their soul that really exists here. The more he thinks about it, the more something about the soul makes sense. Maybe he's just working himself up on the thought.