One thing Leonard's always keen to remind himself of is the story of Sammy Jankis. Although it's told as a cautionary tale (don't make unordered notes, have a purpose), it's actually the key to Leonard's past. When he worked for an insurance company before the attack, Leonard dealt with a claim by Sammy and his wife following a car accident which left him with a near-identical condition to the one Leonard would develop. Tasked with figuring out if it was a scam or not, Leonard ultimately concluded, through Sammy's inability to form new habits through conditioning, that the problem is mental, not physical. The story ends with Sammy's wife, unable to cope with the cost and emotional strain, killing herself in the ultimate test; repeatedly asking Sammy to administer an insulin shot. As the final scenes of the film reveal, however, this isn't totally Sammy's story. After the onset of his memory-loss it was Leonard's wife who couldn't cope with the changes in her husband and orchestrated her own death; is was Leonard, not some John G. who killed her. We know Sammy did exist in some form - we're told he didn't have a wife, implying his case (and Leonard's role in an insurance firm) are to some degree real - so this is not a totally fabricated story, rather another distorted memory. Throughout the story Leonard has talked of conditioning, stating how you can teach yourself something through repetition rather than memory, which is exactly what he's doing in repeated the to everyone he meets (Jimmy's last word, "Sammy", is likely an attempt to appeal to Leonard). What's really interesting is that the insulin murder is an example of a new memory, suggesting Leonard's conclusion that physically Sammy should be able to make new memories was right; Leonard remembers killing his wife, but he's managed to condition himself through repetition it was a story he witnessed, rather than was involved in.
