William James (1842-1910) Wikipedia
William James says that sometimes people who want to dismiss or belittle religious experience use medical materialism as a way of "discrediting states of mind for which [they] have an antipathy." *
* Varieties of Religious Experience
So what is medical materialism?
The concept of "medical materialism," so well described by James in his Varieties of Religious Experience, is just one more form of reductionism that is sometimes applied to matters of religion. For our purposes here, we will define "reductionism" as any attempt to explain the greater in terms of the lesser, that is, any attempt to explain something large and deep and complex in terms of categories drawn from something simpler and smaller and easier to understand.
Reductionistic explanations do appeal to some people because they make something complex seem as if it can be understood in terms of something smaller and simpler to grasp. If we want to belittle the love that exists between two people and say that it's really nothing special, we might say that their "love" is nothing more than the actions resulting from hormones acting on their brains; or that their happy marriage is nothing but a mutually selfish relationship of economic convenience for both of them. What this does is attempt to reduce (hence the term "reductionism") the significance and meaning of something by ascribing it to more trivial causes. It is trying to explain the greater in terms of the lesser.
Some authors have referred to this kind of thinking as "nothing-but-ism," and medical materialism is just one more form of nothing-but-ism. "That religious vision George had was just a result of insomnia," we might say. Or "Mary's experience of existential angst is just a result of a hormonal disturbance."
Here's the way James says it in VRE:
Medical materialism seems indeed a good appellation for the too simple-minded system of thought which we are considering. Medical materialism finishes up Saint Paul by calling his vision on the road to Damascus a discharging lesion of the occipital cortex, he being an epileptic. It snuffs out Saint Teresa as an hysteric, Saint Francis of Assisi as an hereditary degenerate. George Fox's discontent with the shams of his age, and his pining for spiritual veracity, it treats as a symptom of a disordered colon. Carlyle's organ-tones of misery it accounts for by a gastro-duodenal catarrh. All such mental over-tensions, it says, are, when you come to the bottom of the matter, mere affairs of diathesis (auto-intoxications most probably), due to the perverted action of various glands which physiology will yet discover.
And medical materialism then thinks that the spiritual authority of all such personages is successfully undermined.
Medical materialism, in other words, is a mistaken way of thinking that James says needs to be shown up for what it is, just a way that some people use to belittle states of mind they don't like or have never experienced themselves.