Descartes argues that we perceive objects not only as existing in the world in any objective sense, but also subjectively. This is because we are, in fact, attentive to the imaginary mind within us, which is separate from the visible world. Let’s take an example: when we eat something, our real sense of taste causes us to attribute the taste of the object not to it, but to our imagination. We actually see the object and sense the taste associated with it, so that we not only taste the taste, but also imagine that we are tasting it. In the dualism of the spirit and body of Descartes, the mind is the same as the real world as a whole, but the world is not identical to the mind as a whole. According to Descartes, we know what we are thinking, because «it is through thinking that our sensations are transmitted through the body system to the tongue and the gut.» We are no longer looking for the whole object, but our body as a whole. The perceived thing is the «inner», system of ideas, how they appear in our mind and how they are experienced by our senses. Descartes thinks of the sensory impression of an object as representing that object as representing that perception.
Descartes’ belief in materialism is simply a belief, and he admits that we do not know how our own consciousness can explain the comprehensible world, especially when it is connected with us in such a way that the material cannot be part of our mind. But to think that we can explain the world using an intangible cause is incompatible with our human experience, so Descartes assumes that there is a material cause. This is what he calls the non-material cause. According to Descartes, it is the cause of the movement of the sun, or the cause of a dream, or the cause of the invention of something, the cause of the universe.
Yet one of the most surprising features of Descartes’ ideas is that the material cause does not seem to have any reason at all, that the purpose of the universe is for the world to be as it is, without any external cause. Descartes does not provide any explanation for how the material cause arises in the first place, and he does not provide any explanation as to why the material cause must cause the non-material cause. However, as I suggested, what seems to be a simple axiom – «there is no external cause», is very difficult to establish in the materialist worldview. Most philosophers at least claim that we do not know how a material cause arises. But from the point of view of the materialistic worldview, no one can know whether a material cause arises in some primitive mechanism, or as a property of the universe, or as something that would arise if there were nothing but material, or as a result of a process, natural selection, or from an unthinking source such as spontaneous generation. There are many different possible answers to these questions, and they may all be correct, but we can never know for sure what the material cause is.