Dan Aykroyd and Harold Ramis, the believer and the skeptic, wrote Ghostbusters together out of a shared borderline-obsession with the occult and the people who have sought to scientifically and rigorously study it. Dan told The Daily Beast back in 2009 that he is a fourth-generation believer and his now-deceased
writing partner, Harold, was a skeptic but as such was just as familiar with studies that Dan had read in fascination as a kid in journals like that published by the American Society for Psychical Research.
The alleged scientists who do the sort of research published in journals like these believe that ectoplasm is the chemical substance that ghost who take physical form are made out of. Though such "materialization" has never been reported, Aykroyd and family still hold out hope of one day seeing an ectoplasmic ghost.