My conclusion on this movie after many views
I've seen this film over 20 times the past 10 years and have caught on to a lot of things each time. I've seen a lot of posts on here about Leonard's past being Sammy's past and what not. After my last viewing, I have come to the conclusion, which makes a lot of sense now, that Leonard IS in fact faking his illness. He does not actually have brain damage and cannot make new memories.
I based this on several things:
1. Even though he "cannot" make ANY new memories, he still remembers certain things, usually about his investigation on his wife's murder such as knowing he has a photograph of everything and that he has an entire file on his wife's murder.
2. Every time he meets someone either new or that knows him, or when someone is about to test him, his memory instantly resets. This kinda shows that he wants to constantly give the perception that every time someone see's him, he cannot remember them. Some times his memory last while, like he was aware of the entire night he was WITH Natalie until he woke up. Other times it only lasts a few minutes, coincidentally whenever someone arrives or *beep* with him, like when Natalie spit in his beer and gave it to him 20 seconds later.
3. The scene where he meets Natalie at the restaurant. If you watch him, he clearly looks right at her as if he knows its her, yet then keeps walking by.
4. Going back and adding to #1, if he cant remember anything after the incident, how the *beep* does he know that he has no short term memory. Some may say the tattoo on his hand, but he doesn't constantly look at his hand. He scene where Natalie takes all the pens and *beep* with him, his memory reset before she got there and after he got there, yet he KNEW he needed a pen to write down what happened.
My take on Leonard is that his wife got raped and murdered and he could just not handle it. He remembered Sammy, who was also a faker, and realized that Sammy did it just as a way to escape life and have people pity him. Leonard, not being able to go one with his life, creates a new persona of one whom cannot remember anything before the incident, yet wants revenge on the murderer. So he teams up with the cop assigned to the case, Teddy, and gets his revenge. Now avenging his wife, and having pretended to not be able to produce new memories so some time, Leonard has absolutely no purpose left in life, so he decides to keep on doing it. Like why did he NOT get that tattoo on his heart after killing the guy? Why did he take pages out of the report? By the time he gets to where he is in the movie, which has been at least a year since the murder, he's been so used to doing the whole memory loss thing that he actually starting to believe it himself, which is why he does things even when no one is looking. Maybe he either wants to keep playing this role, or he just wants to forget the things he has done since the murder.