Board Thread:Wiki Discussion/@comment-31960778-20170721040405/@comment-32182236-20170903125154

Papyrus simply didn't make as much of an effort to remember as Sans did. He probably didn't realize that he could easily forget about Gaster, being in a timeline where Gaster never existed. It was so easy to forget about Gaster that Sans needed a picture of Gaster and his followers in his lab with the words "don't forget" to not forget about Gaster. (Sans was there too. Neither Alphys not Papyrus was, though, as the other people Frisk doesn't recognize.) So what about someone who has neither of those things? Also, while Gaster is Papyrus' origin, Papyrus did NOT work with Gaster in his experiments like Sans did. He probably didn't even know about Gaster's dissapearance.