"Well... it's finite, but at the same time it can replenish itself," said Kayneth, trying to think of an appropriate comparison. "It's... a bit like how if you lose a respectable but survivable amount of blood, your body will be able to replenish the supply, but you'll be left a bit weak and groggy for a while. With that said, there is something of an external element to it, as there are two kinds of energies that can be converted into prana-mana and od. Mana is everywhere, while od is produced by living things. While it's technically possible to use od as your only source of power, it's not recommended, since it's far less abundant than mana. I'd compare them to actual firewood and kindling respectively-it's difficult to start a fire just using a log, and it's difficult to sustain a fire just using kindling."