As I mentioned earlier, Descartes did not adhere to the idea that the mind is a separate substance from the body. This is a decisive moment in Descartes’ understanding. When he discussed mind and body, he insisted that there was a difference between the two. He believed that the mind is a rational thing, but inseparable from the body. Rather, the body is an effective cause, a tool that the mind uses to achieve its goal. «The relationship of things to each other is always determined by the nature of things. It is intelligence to produce what is in the mind, what makes them change, what makes them exist». Mind and body are intelligent things, but not separate substances. Descartes believed that although we can think of mind and body as if they were separate substances, mind is the effective cause of the body. The mind is the inner force that exists in the body and through which it is built. An essential part of Descartes’ understanding of mind is that mind and body are not separate substances. This is not to say that mind and body are separate substances. They are one and the same substance. In a sense, the body is simply a tool that is used to help the mind achieve its goal.

These are words that we know come from the Latin verb rerum (asire in English). Feelings alone, and also only imagination, will not do anything, neither see, nor hear, nor think, but must be connected together with the intellect (lat. Intellectus), and they will be ready to produce sensation. For example, if you want to find out whether there are two things or not, you have to reason to do so: the mind, which is of the same nature as the imagination, will cause the sensation of the existence of two things; because a reason alone, if you set it up to conjure up a sensation, will cause both sensation and doubt; but doubt alone has its own nature. The difference between existing and non-existent things is this: every thing that exists has a cause, and every thing that does not exist has no reason. Therefore, there is a reason for everything that exists, and there is no reason for everything that does not exist. So, this reason has existed from eternity; for each cause, it is already present and remains after a series of things: for example, those things that were caused come to their end and die. Therefore, this cause of things exists from eternity, and this cause of things does not exist only from time.

In the second part of the Treatise on Man, Descartes develops the concept of reason, which combines reason, imagination and thought. This is the first attempt by modern philosophy to develop a theory of mind, because, in addition to believing in what is believed to be true, the object must also judge whether it possesses that property. Descartes’ theory does not attempt to create an object called the «comprehensible world» that does not depend on the mind that perceives it, but rather represents the mind’s perception of the transcendental mind.

In his earlier Reflections books, Descartes addressed epistemological questions about what knowledge is and how we acquire it, and how knowledge about external objects can be obtained. In the third book, he presents the idea of an inner mind (intelligible world) for solving the problem of cognition in the physical world. As he notes: «So, if I know this, I must also know that I do not know it; and if I know that I do not know this, I must know that I know it».

His solution to the problem of epistemological knowledge is that mind exists somewhere, which is itself a judgment, and that mind itself is a single judgment. This is the mind, and the mind is the domain of knowledge. Descartes’ theory of mind as the mind behind the world is to some extent a complete interpretation of the «mind behind the world» of Pythagoras, but Descartes’s theory is still sufficiently related to the idea of Pythagoras to qualify as a form of mind dualism and body. The story of how we can find out what is in the mind of the world in Reflections can be seen as a continuation of this.