Julien de La Mettrie is considered one of the most influential determinists of the eighteenth century. Along with aiding the furthering of determinism he considered himself a mechanistic materialist.
He believed that mental processes were caused by the body. He expressed these thoughts in his most important work Man a Machine. There he also expressed his belief that humans worked like a machine. This theory can be considered to build off the work of Descartes and his approach to the human body working as a machine. La Mettrie believed that man, body and mind, worked like a machine. Interestingly, although he helped further Descartes' view of mechanization in explaining human bodily behavior, he argued against Descartes' dualistic view on the mind. His opinions were so strong that he stated that Descartes was actually a materialist in regards to the mind.
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 Huygens is to a watch of Julien Le Roy". 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.
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