Ancient texts talk about root canals, skin grafts and all types of fundamental medicine but why is it we don't have any old texts about people coming back from war damaged. Even if they didn't know about brain chemistry or the like, surely people would pick up on the repeated symptoms over thousands of years.
This is literally the most frequently asked question on this subreddit, which means it's been covered ad nauseam. Please check out our Very Frequently Asked Questions section, particularly this Monday Methods post.