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.
Descartes begins the fourth book with the argument that, although the world seems absolutely solid, the principles governing all the complex phenomena that we perceive around us are not rigidly fixed. They are constantly being modified or transformed. As Descartes says in his article «An Essay on Human Understanding», the sun seems to move across the sky because the changes that occur in the fixed world that we perceive in general includes the movements of the planets and stars, and what he could describe these changes, as a continuous movement, would mean that the underlying substance is subject to constant change, and therefore is not absolute. Instead, the only substance that is not subject to constant change is the mind that we have about ourselves, the mind that is material. This mind is an understandable world, but not everything is clear, and those things that are incomprehensible are called «beings» or «active principle». The idea that mind or some other immaterial, non-causal principle should be responsible for the world that we perceive around us has been called «metaphysical materialism» or «materialism from within».