Ah, you mean in an edit. 

You are actually writing two posts when you edit. Three if you edit again. 

Lets say you post and a minute later, you edit. The edit will be another post, 20 blocks (60 seconds) after the first, but linked to the first, like a history. Everything is saved. When you edit, you aren't actually overwriting the original, just what the interface pulls first. It would be possible for an interface to include an entire history of a post - but there is no call for it. If this was a company though, there would be a call for the history to be accessible :)