Board Thread:General Discussion/@comment-27524978-20160210030742/@comment-4383275-20160502162904

Stevethebarbarian wrote: Well, just for the sake of argument, if Sans really was Ness, then how would he know there are no PSI abilities in the distant past? It seems reasonable that he would bring the most useful defense object he had in order to survive an aincient world he knew absolutely nothing about.

It's not an "ancient world." Earthbound takes place in 199X, while Undertale takes place in 201X. This also explains why Sans looks older than Ness.