That depends on what you mean exactly.
As you are using the word pensieve, I guess you mean observing someones memories. It's been a while since I've studied up on the brain, so I might be misremembering stuff(kind of ironic, given what I'm writing about) or it might be outdated, but I recall reading studies that suggest that we don't really forget stuff, it's just that we don't have access to it anymore. So given enough research we might obtain a way to view memories. Thought the issue of how reliable the memories are still remains, as stuff tends to get simplified and other processes might muddle the memory. Of course, there is more to it, but I don't want to write a wall of text and don't think I know enough to do a good job of it.
The other option would need some form of time travel, as to the possibilities of that, I'd rather not speculate, as it requires an understanding of physics that is hard to simplify.