Like Socrates, Plato and Augustine, Descartes was a rationalist. Certain knowledge can only be the result of reason. The more reasonable something is for our thinking, the more likely its existence is. A rationalist believes that something clear and plausible for our reasonable mind must be true. He has faith in the nature of thinking. Descartes explains his faith with a perfect being, who’s presence he can experience when thinking. The nature of this being includes the certainty that it is real. What we recognize with a reasonable mind, like the mathematic relations in the world, must therefore reflect reality. Old books on the other hand, and even sensory perception, can be either not reliable or incomplete. Besides Descartes, there were other important rationalists in the 17th century, like Spinoza and Leibniz. They all ask the question what we can know, and how certain we can be to align with reality. Additionally, the relationship of body and soul is an important question that receives significant attention.
A popular opinion at the time was that mankind must simply accept that there are no reasonable answers to the big questions, that there will always be uncertainty around it. Some philosophers supported this to leave room for religion and belief. But Descartes did not accept this. During his lifetime, a method based on precise observation and careful curated experiments was established to achieve an understanding and description of nature. Many laws of nature were found and mathematically described, defining the behavior of objects. The physical world was captured with mechanical laws, and that resulted in the attempt to trace living objects back to the same laws. Could plants, animals and humans be the results of mechanical processes? The body of a living organism consists of the same material as the lifeless things described by physics. And it is subject to the same laws. But philosophy is not just about the body. It is also interested in the soul, and Descartes wanted to find a secure path to knowledge for all aspects of reality.
Descartes had difficulty to differentiate dream and reality. He did not fully trust his sensory organs. He was however certain of his doubt, and therefore certain to be thinking. He was certain to be a thinking being. The thinking self is more real than the sensory world to him. From this point he continued to work. He wanted to develop a method for investigating the soul that is just as precise and and reasonable as modern science is for investigating the objects of the spatial world. His ambition was to philosophize with mathematical method and precision. Like a natural scientist, he tried to split up a question into several smaller ones ant then proceed moving from something more simple to the more complicated.
This article does not differentiate soul and spirit. The initial meaning of both words is breath of life. In the 17th century it was popular to try and explain living organisms, including humans, as a purely mechanical processes. Massive progress was made and much effort was put into the explanation of mechanical processes. But how does a soul fit into that worldview? Is it needed still? Can something spiritual have an effect on a mechanical process, or the other way around? Both Plato and Descartes clearly separate body and soul.
Descartes believes that when we think reasonable, this happens in our soul and not in our body. He considers humans to be a dual being, having both a body and a soul. That is what makes the difference to an animal, which he considers to have just a body. Animals are therefore driven purely by the mechanical laws. That is because the body belongs to the spatial or material reality. It has expanse, takes room in space. The soul on the other hand is part of the spiritual or conscious reality. Descartes is a Dualist, he divides the world in expanse and thinking. Descartes considers the soul of a human to be independent of the body. It continues to exist when the body is gone. While alive, the body interacts with the soul. The soul is constantly confronted with sensations that originate in the needs of the body. The goal is for the soul to take control. It shall turn towards reason without getting distracted by the preferences and sympathies of the body. That is how we find truth. Because when we think with reason it is not a process of our body, but we do it with our soul.