In the meditations, Descartes resolves to doubt all that may be doubted. He arrives at the proposition which he thinks it is logically impossible for him to doubt. What is this indubitable proposition? Is he correct in thinking that it cannot be doubted? Why is this proposition so important in his epistemology?

Descartes resolves to doubt all that may be doubted in order to uncover certain indubitable truths. Although he was not a sceptic he used techniques used by sceptics in order to doubt and question long held assumptions of knowledge. The quest was to find knowledge that had a strong belief condition, indubitably and was a priori knowledge. That is, knowledge that could be logically proved and not dependent upon experience to terminate the seeming infinite regress of reasons.

Descartes' rationalist foundation philosophy was based upon necessary truths, knowledge that could not be logically challenged. His introspective Meditations arrived at the famous indubitable proposition of 'I think, therefore I am' or 'Cogito ergo sum'. By this proposition he believed he had come by knowledge that could not be sceptically challenged.

Descartes' proposition depends upon his belief in the duality of existence, that there is a mind or consciousness that is separate from the body and the rest of the external world.

In questioning every proposition or item of knowledge Descartes has had to doubt every item of knowledge he believes has come from his senses - 'it is prudent never to trust entirely those who have once deceived us' [First Meditation - pp96, Penguin edition.] The rest of the propositions are prior to experience and can be proved mathematically or logically.

Descartes often thought that the mathematical sciences were the purest form of knowledge, as they did not depend upon the external world perceived in a very deceiving way.

His technique of doubt has brought him into questioning the external world that includes his bodily senses. So where does the mind or Cogito come from? The rationalist foundation that Descartes is trying to establish depends upon rather speculative tenets such as the mind is not a material object. In order to disassociate more from the external world that could deceive it Descartes also postulated that the mind does not have imagination. How can Descartes prove this?

That Descartes thinks assumes that thinking is his experience. An all-powerful, deceitful devil may 'own' this experience. Though why an all-powerful demon would bother to deceive a 'non-existence ' is beyond me! Ownership also assumes self-consciousness. Today without this self-awareness computers would be thinking, sentient existences, when the thinking is owned (if not copyrighted) by humans.

To stop the proposition from becoming closed and barren Descartes has had to establish that it is a fact from the world. That it is both a priori and synthetic.There is a link between the mind and the external world. In doing so he has opened up the sceptical regression by considering the existence of God as a main principle for establishing the proposition as one that can lead to other facts. Empiricists would claim that you cannot mix synthetic and analytic propositions.

If Descartes is not thinking, does he exist? Can he establish that in a sleeping state he is also dreaming, and so is thinking? What if he was knocked unconscious, as he may have been during his time with various armies?

The laws upon which Descartes founded his proposition may well be infallible, but are, unfortunately operated by fallible minds. Descartes has tried to bind the laws of knowledge with the laws of logic. This has meant that propositions have to be bound by logical certainty, and does not give us scope to establish knowledge that may well be logically doubtful, but not wrong.

The proposition is so important as a foundation for the rest of his theory of knowledge. With this one proposition Descartes hoped to build further indubitable propositions.  It would also terminate any further regress of reasons by sceptical criticism. Its truth does not depend upon any prior assumptions. It is unconditional knowledge based upon mathematical reasoning and so is not based upon doubtful perceptions. To Descartes thinking it also proves existence. It steers round the thorny issues of senses, dreaming and existence.

On its own 'Cogito ergo sum' cannot tell us anything about the external world. That Descartes exists in mind, but not necessarily in body, stop any further logical propositions stemming from the Cogito from ever gaining knowledge about the external world. In order to enable the proposition to be fruitful Descartes has had to span the gap between the material and the immaterial, Cartesian dualism, by proposing the existence of a non-deceitful God. Descartes proposition also depends upon the fact that it is both a priori and synthetic, a proposition that in itself is doubtful.