The "Cartesian evil spirit problem" is the realization: isn't it possible that the world around you is an illusion and your mind is just being fed these perceptions by a spirit (or a neurologist if you prefer) who wishes to deceive you? To many people this sound silly at first, but what exactly is implausible about this?
I believe Descartes himself ended up "resolving" it with something like "god is good and he wouldn't allow me to be in such essential error". But it's painfully clear that argument doesn't work even if you believe in a Christian-like God. God does allow that. Look at the world around you and tell me whether there are people called 'materialists' who believe that God doesn't exist and that everything is material. Your God's not stopping them from being in such essential error. He could prove his existence to them, but he doesn't. So why would he treat you any differently?
It's clear that the possibility can't be logically excluded, but it doesn't need to be. The fact is the other people in the world might be real and our choice is between acting as if they're real (in which case how we treat them matters) and acting as if we're alone (in which case morality becomes almost entirely moot), and therefore we morally must behave as if they're real.