Primarily he considers existence of the external world and whether our experience hold knowledge of this world or whether this knowledge is merely an illusion. He makes it quite clear how misleading some of external sensations can be. We are never sufficiently aware of subjectivity of our own thought and senses. The only thing we directly experience is the nature of our own ideas and we do not realise how our own appreciation of certain concepts may be very different from the objective character of the external world. Descartes takes a look at memory, imagination, hallucination, dreams, predictions, etc. which he calls our (sensory awareness) as these are part of the way we perceive the external world, he doubts at first that any of these internal experience holds any truth or existence. As he is very sceptical he raises the problem whether any of these given experiences contain truth or objectivity at all. Since we never have the chance to stand outside our own perception, it is impossible to contrast it with the external world.
Descartes is hopeful to prove subsistence of the external world (physical objects located in space), and so he returns to a very basic stage and acknowledges the existence of minds as an immaterial substance and God. He then accepts that matter exists as long as it is not a projection of his own mind or God. As Descartes previously established the existence of God as a perfect being, he therefore has concluded that God is not a deceiver. This very clear concept leads him to accept his clear and distinct sensory experiences are a result of external objects of material nature. Once these corporeal things (objects of a tangible, material nature) can be considered as self-evident ideas, they can no longer be products of the mind or God.
Right in the beginning Descartes makes a very clear distinction between imagination intellectual thought. He states that intellectual thought is abstract and without shape, Descartes would call it extension, while imagination is perceived in pictures “the eyes of the mind” (mental viewing of the external world). Another way in which Descartes proves the existence of material substances is by showing that pure intellect is totally controlled by will and therefore anything can be easily ignored. While imagination is not controlled by will and this concludes that, as it is impossible to restrict imagination, matter must exist, as without the subsistence of things to imagine there would be no imagination.
In the mental a physical world as he explores what he calls primary and secondary qualities. The primarily qualities are to Descartes the more trustworthy mental perceptions and reality. The secondary qualities represent the not so trustworthy physical reality. He explores this relationship as he considers connection of the state of the body and physical sensations. I believe at this point Descartes makes some contradictory statements, as it seems he is not completely sure of the relationship between the two and their significance. This problem might be explained as medicine was not yet so advanced in Descartes time and therefore many facts about the very close relationship of bodily state and physical sensation were not known yet. Another thing that contributes to this is that Descartes was a strong Dualist and subsequently believed mind and body to be very separate entities.
An addition Descartes questions the origin of dreams. He argues that sleeping and being awake can be very easily segregated; as when one sleeps (dreams), one does not perceive anything one could experience in the state of being awake. He believes that when we dream it is an entirely different faculty is at work. Nothing one dreams originates in what one experiences when awake. He proves this by saying that dreams are never linked by memory to other actions and happenings of life itself. Obviously today medicine can prove quite the contrary, but at Descartes time this theory was quite acceptable.
Descartes does regard the brain to being of critical importance concerning perception and sensation. He accepts that the brain receives, interprets and replies all nervous information. This information is collected by extensions of the brain; organs which act as receptors. They are in a way a sensory extension of the brain. He adds to this that nothing can assist better to preserving the body than what the mind itself feels. Still this mind and body experience can be deceiving, as sometimes what one feels is right or preserving for oneself is actually not the case at all. “Thus for example, the pleasant taste of some food in which poison has been mingled may invite me to take this poison and thus to be deceived.” Here we gain states how easily we can be deceived by our senses. And he compares the healthy human to a malfunctioning clock. “The clock is destined by its maker” This agreement he says may hold some truth, still he states it would be absurd to consider an ill man unworthy of a healthy individual.
What is slightly contradictory about Descartes argument is that he considers the mental faculties to be more real, and at the same time reveals the proof that sensory information (imagination) has more significance and strength on the mind than pure intellectual thought.
Right in the beginning of this last meditation he is already trying to prove the existence of external object, one way of achieving this is by recognising the distinct ideas, Descartes had of external objects, they were thoroughly imprinted in his memory, he realised that the concept of these ideas could not have originated from his mind. Therefore holding the clear knowledge of these objects was a projection of other objects. He realised he had no description of these objects except what they themselves reflected subsequently these objects resembled the ideas which they caused. From here Descartes steps further and claims that since all ideas in his mind derive from through sensory experience he held no concepts in mind which had not previously passed through one of the sensory organs. After this section he again returns to argue his dualist ideas. As a result he restates the cogito (I think therefore I am, lat.: cogito ergo sum). Still at this basic stage he is in a state of solipsism and all his essence consists in that he is a thinking being. Further he states that he has a body he is attached with closely. He reduces his body to be an extension of his mind but without thought or intellect. He pursues this strong dualist argument that he (his mind) does not need his body to exist. He also says that the body cannot without intelligent substance attached to it. He compares this mind-body relationship with a pilot in his ship, as they are two very close entities, although the body cannot exist without the mind, like the mind can without its extension.
Descartes sixth very conclusive meditation has similarly to the first very basic concepts. What makes Descartes meditations so remarkable is his methodological approach to his philosophy. He starts right at the foundation of knowledge building his own basis and thoroughly working himself upward. Even though some of his thoughts are slightly contradictory he still manages to make them plausible, and one can accept certain incongruities as one considers the time and social structure he was living in. I believe the fact that Descartes returns to the doubts raised in the first meditation bring more sense to the rest of the five writings and in a way holds them together. He very effectively uncovers his dualist ideas but he does not exaggerate or make them absurd as he also continually reminds us of the great importance of the mind and body relationship. Most importantly he proves his existence and the existence of others which is considered as one of the main problems all philosophers are faced with.