Sure, and the witness could have been paid off, hallucinating at the time, memory wiped, mentally unstable, anything really. If there was no corroboration as far as a spirit or soul goes... then there would be no organized religion. Take Christianity for instance... if Arcanus says something: you usually agree with it. That is corroboration with Arcanus's statement. A soul or spirit should be no different. 
Please give me a home where cloud buffalo roam
Where the dear and the strangers can play
Where sometimes is heard a discouraging word
But the skies are not stormy all day
