Board Thread:Theories and Speculation/@comment-29769110-20170115161829/@comment-734582-20170117205747

Lorenzo.r.1st wrote: I know.. I was just jokin.. Itachi used it while blind, and madara used with no eyes at all.. which makes me kinda pissed, cuz they never give us a good explanation for it.. As Tobirama's explanation showed, the source of the Sharingan and it's power is in the brain. Susanoo needs both eyes to be cast initially, but once cast it can be generated by the brain itself and the eyes are no longer needed.

From mythology, Amaterasu and Tsukuyome were born from the eyes and Susanoo from the nose.

As to why it needs both eyes first, one can see the chakra being sent to the eyes as a signal, and the whole signal is needed to awaken Susanoo. Obito sent the signal to Kakashi's eye, but once there it lost connection. the piece of chakra he needed for Susanoo was locked in Kakashi's sharingan eye.