Board Thread:General Discussion/@comment-27524978-20160210030742/@comment-4383275-20160730175244

Tabbender wrote: The lack of PSI is actually the only explanation, since Sans can't manipulate matter (and Ness' powers are much stronger, so he would have used them against Frisk in the genocide route). But then where does his powers come from?

I'd suggest they come from W.D. Gaster. That would explain Sans' use of Gaster Blasters, as well as the other in-game evidence linking Gaster to Papyrus and Sans.

Tabbender wrote: Also, if Sans was human, Asriel would have 7 human souls during his fight, and when he releases the souls, you can see there's only 6 human souls. Oh and he would have determination (logically). And Ness' durability don't seems to come from PSI either. As mentioned previously, Sans being a revenant removes all human-related plot holes from the equation.