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.