Our power is in our souls. The proof is this incredible anime.
The main character, Kurosaki Ichigo, searches for his power at the bottom of his soul and always manages to find it. The shinigami's main source of power is himself depending of how big is his "reatsu" (spiritual power). Some reatsus are really huge...
Lets get back to Ichigo's power. There are not enough episodes yet to make any conclusions but I think that Ichigo's reatsu is the only one that can match Aizen's... Lets together watch the anime to the end and find the truth :)
Translated from Bulgarian language by: Ted^Maverick (TV_Squad)