It turns out Jason Bourne didn’t really have amnesia. That would require a hit on the head or something similar. He would then lose all of his past memories and kind up wake up clueless, maybe even unable to make new memories.
No, Jason Bourne had selective amnesia where he was able to forget all the bad things in his life, but remember how to speak several languages, fight 16 bad guys at once, and generally act like a superhero. This is called ‘dissociative amnesia’ which usually occurs after a traumatic event.
So, it is a form of amnesia just not one that requires you to be bonked on the head. It’s sort of the brains way of dealing with something to hard to handle. You forget that incident but remember pretty much everything else and function normally.
It is the perfect writer’s device. Start your character with nothing but an awesome set of skills and bad guys to foil…fill in the personality later.
More on this from an engaging post on neuroscience, The Weird History of Amnesia:
The major fascination with amnesia is that it’s so specific. When an amnesiac wakes in a hospital, they may not know who they are or where they are, but they do know that they are in a hospital. They know what hospitals are and what they look like. They retain the ability to talk, to count, to recognize certain aspects of the world they live in, while blanking out personal memories entirely.