User blog comment:Garhdo/Extended Cut - opinions?/@comment-1441969-20120626205409/@comment-1440333-20120627082513

Synthesis has all ready been accomplished in a way previously in the series. Don't believe me? Shepard. The man died. He was a corpse, and if your religious then his soul would have moved on.

And yet he is brought back to life with all sorts of synthetic pieces wired into his body and DNA to restore him to life. The Catalyst simply says that after seeing it done with Shepard the Crucible can make an improved version of this synthesis possible for everyone in the galaxy.