Mark Olson delves into demonology, and questions whether modern science has all the answers. He makes four major claims.
To start, Olson defends the past, writing that ancient folks weren't stupid for believing in demons, and furthermore, their lack of technology was offset by their "keen or keener understanding of human nature." Since no one here is arguing that people who believe in demons are stupid, I'll set that straw man aside, near the roaring fireplace of reason. (I'll also set aside the unevidenced claim that ancient folks' insight into human nature was keen or keener, since it begs the question.)
After dismissing a gnostic view of the body/soul dichotomy, Olson claims that not only are body and soul unified, but anti-psychotic drugs work because demons are subject to physical laws--the spiritual entity can be trapped by physical constraints. Besides, medicine can't really explain why some people are sane and others go crazy. Lastly, and consequently, open-mindedness about the spiritual realm is a scientific virtue.
Let's look at each point in turn.
First, from a scientific perspective, saying that demons are bound by the laws of physics initially sounds reasonable. After all, they would need a substrate for their wiles, right? But parsimony becomes a problem. If demons, for the longest time, have necessitated exorcism for their removal, why is it that all of a sudden we can control them with the right drugs? Does "demon possession" really explain anything about typical possessed behavior, at least that which has been observed by disinterested parties? (Accounts of levitation, telepathy, and the like, which would seem to be knock-down evidence for supernatural hanky-panky, are notoriously unreliable.) "We battle not against flesh and blood," the Bible claims, "but against powers and principalities." Nowhere is it mentioned if these principalities are in any way bound by chemistry or physics.
Second, does possession explain why some people are sane and others are crazy? At least with biochemistry and genetics we have a shot at an answer. We can search for a chemical imbalance, a tell-tale virus, a genetic marker. On the other side, only an exceedingly rare number of persons are possessed, and for reasons utterly inscrutable. As I've noted elsewhere, occult activities are often blamed--but in the best-known modern instance, the victim was a devout Catholic.
Third, open-mindedness is indeed a virtue, scientific or otherwise. But the evidence for actual possession is so rare, and so poor, that skepticism is by now the default position--even for the Catholic Church, the standard-bearer in the War on Evil.
Last, an observation. Since even secularists like to use metaphors of possession ("dealing with personal demons," "I just wasn't myself today," "I don't know what got into me"), it seems that the literal existence of demons is secondary to their existential import.
Sleep is a virtue, too, so I end here. I welcome criticism, since this post could use some refining.