There were a number of implications to Materialism that meant it was a deeply heretical ideas during the enlightenment. From what I can tell, it is excluded from the enlightenment as a school of thought by some historicans.
https://en.wikipedia.org/wiki/French_materialism
It's not an area I know well, but one I've looked into in recent weeks as an early influence on Marxist belief. I'm just going to copy and paste some stuff from Wikipedia, to give you an idea of what its like. it is the sheer radicalism of materialism that means it is somewhat taboo, even before Marx, because of how far it went to reject theological understanding of man and morality. They might not sound so radical now, but this was pre-Darwin and the evolution/creation controversy in the 1740's. Christianity was still dominant along with the belief in the soul, original sin, etc.
I believe many Conservative Christians would argue that the libertinism of materialism, and its hedonistic imperative led to the ideas of the
Marquis de Sade, (who is sort of a spectre in the background when you're challaging christian morality and became a by-word for evil and sexual perversion and gratiutious violence in sadism) but there isn't an obvious relationship beyond the affirmation of sexual desire/perversion as natural and the implications it has for morality.
Below is just one example, but materialism represents a series of challanges to Christian morality.
Julien Offray de La Mettrie is best known for his work
L'homme machine ("Machine Man" or "The Human Mechanism").
Man and the animal
Prior to
Man a Machine he published
The Natural History of the Soul in 1745. He argued that humans were just complex animals. A great deal of controversy emerged due to his belief that “from animals to man there is no abrupt transition”. He later built of that idea claiming that humans and animals were composed of organized matter. He believed that humans and animals were only different in regards to the complexity that matter was organized. He compared the differences between man and animal to those of high quality pendulum clocks and watches stating: "[Man] is to the ape, and to the most intelligent animals, as the planetary pendulum of Huyghens is to a watch of Julien Leroy.". The idea that essentially no real difference between humans and animals existed was based on his findings that sensory feelings were present in animals and plants. While he did recognize that only humans spoke a language, he thought that animals were capable of learning a language. He used apes as an example, stating that if they were trained they would be “perfect [men]”. He further expressed his ideas that man was not very different from animals by suggesting that we learn through imitation as do animals.
His beliefs about humans and animals were based on two types of continuity. The first being weak continuity, suggesting that humans and animals are made of the same things but are organized differently. His main emphasis however was on strong continuity, the idea that the psychology and behavior between humans and animals was not all that different.
Man a Machine
La Mettrie believed that man worked like a machine due to mental thoughts depending on bodily actions. He then argued that the organization of matter at a high and complex level resulted in human thought. He did not believe in the existence of God. He rather chose to argue that the organization of humans was done to provide the best use of complex matter as possible.
La Mettrie arrived at this belief after finding that his bodily and mental illnesses were associated with each other. After gathering enough evidence, in medical and psychological fields, he published the book.
Some of the evidence La Mettrie presented was disregarded due to the nature of it. He argued that events such as a beheaded chicken running around or a recently removed heart of an animal still working proved the connection between the brain and the body. While theories did build off La Mettrie's, his works were not necessarily scientific. Rather, his writings were controversial and defiant.
Human nature
He further expressed his radical beliefs by asserting himself as a determinist, dismissing the use of judges. He disagreed with Christian beliefs and emphasized the importance of going after sensual pleasure, a hedonistic approach to human behavior. He further looked at human behavior by questioning the belief that humans have a higher sense of morality than animals. He noted that animals rarely tortured each other and argued that some animals were capable of some level of morality. He believed that as machines, humans would follow the law of nature and ignore their own interests for those of others.
