In his second meditation, Rene Descartes reveals and tries to verify the proposition "I am thinking, so I exist". As demonstrated in his first meditation, a proposition may only be made true if one cannot conceive of any situation where it may be made false. If one is able to have any reason to doubt the reality of any object, than that object is automatically considered to be dubitable and cannot be taken for full value as it may not even be what it appears to be. However, if one can be certain of something in every situation, it is considered indubitable, and by Descartes would be considered foundational, meaning that they are the basis of what our knowledge is formed upon.
The proposition "I am thinking" is made indubitable by considering the fact that even if one tries to believe they are not thinking, they are still thinking in an attempt to make such a proposition false. As well, Descartes points out that even if one is to be deceived of everything around him, one can still be certain of their own existence. Whether it is by dreaming, hallucinations, or a deceptive evil genius, one can not experience something unless it in fact exists in some world. Although it is possible to experience such things even when they are false, in order for one to conceive of such things they must be in existence somewhere. With this in mind, one can not honestly conceive of any situation where they themselves don't actually exist. In order for one to experience any sort of sensory or cognitive experience, one must be in existence to do so. Descartes argues that this is what makes one's own existence indubitable. This leads to the conclusion that if one is thinking, then one must be in existence to think, therefore making the proposition "I am thinking, so I exist", indubitable.
On the other hand, the proposition, "I am walking, so I exist", would be considered dubitable by Descartes" prescribed rationale.