Descartes introduces us to his doubt with a recollection of being "struck by the large number of falsehoods" he had believed in his youth, and "the highly doubtful nature of the whole edifice" he had based on them. He concludes from this, he tells us, that it is necessary once in his life "to demolish everything completely and start right again from the foundations" if he is to establish any kind of stability, here taken to mean certainty.There is an immediate problem with the project evinced here, and that is shown through the use of the building metaphors. In the first place, Descartes sees the falsehoods he believed at a young age as the foundations on which he based a doubtful edifice. The solution to this, he considers, is to "demolish everything" and "start right again from the foundations". The problem here, is that it is unclear whether he is demolishing everything, as he suggests, and if this is the case, why any foundations remain. To narrow the scope of his scouring of his beliefs for falsity, he says that "Reason now leads me to think that I should hold back my assent from opinions which are not completely certain and indubitable". In order to quicken the process, he aims straight for the foundations of his beliefs, since finding a flaw there would rule out a whole subsequent edifice. He goes on to say that the senses have either been the source or the means by which he has come to believe what he has accepted as most true, but since he has apparently found the senses to have deceived him, he considers it wise not to trust completely those who have deceived him even once. The immediate problem here, it seems, is that it is not imprudent to trust someone after they have deceived you, if you can (as in the case of senses) understand why they may have done so, and in which circumstances they may be reliable or not, but this is not enough for Descartes. Any doubt rules out the senses altogether. Descartes, at least in this sense, is remaining consistent with his earlier decision to rule out anything which shows itself dubitable. True, he said this about opinions, rather than a particular means of receiving opinions, but if the means are dubitable, for Descartes at least, so are the opinions received therefrom. However, there may be more of a problem here than at first appears, and this certainly becomes clear later in the Meditations. Descartes says that the senses have deceived him, but it is not clear how the senses could do this. Descartes undermines the ordinary mind-set of taking sense perceptions for granted by referring to the examples of his dreams, in which he likens his condition therein to that of a madman.
Dreams, presumably, then give us an example of the senses deceiving someone. There seems a problem, though. Deception seems a matter of making a bogus truth statement about events. It is far from clear how the senses would be in a position to make truth statements about sensory data along with simply presenting that data.Descartes himself hints at this problem, although apparently unawares, when he says "I have been tricked by exactly similar thoughts while asleep!". Here he describes it as the thoughts about the data which mislead him, and this seems more truthful, since it seems to be his interpretation of the senses' information which is wrong, shown by the possibility of lucid dreaming, for example. Interestingly, Descartes suggests some sort of understanding of the idea of the material of the senses being inherently true or at least devoid of truth value, when he goes on to describe the materials from which composite creatures are made being true, regardless of the truth or falsity of those composite creatures. This is an interesting question, but it seems again to avoid placing blame where it belongs, on the reasoning participant as misinterpreter, and the point seems to be that these fundamental materials do not require interpretation, but rather simple experience. Descartes, however, does not seem to notice this. Descartes attempts to carry on the process of doubt, but struggles with his habitual opinions, which are seductive, and to counter-act his tendency to believe them, he throws himself whole-heartedly against them. Rather than simple doubt, from a position of neutrality, we have an active belief, pretentious though it is, against his former opinions. This amounts to the hypothesis that a powerful demon is misleading him through his senses. I consider this hypothesis to be all-important to Cartesian doubt. I think it defines it, perhaps much more than it ought to. I would argue Descartes is not from this point on looking for what can be doubted, but rather what must be true given this hypothesis. Descartes gives a list of the qualities he must have as a thinking thing, for example. He must be something "that doubts, understands, affirms, denies, is willing, is unwilling, and also imagines and has sensory perceptions". I believe Descartes gives this list because these are the characteristics necessary for him to be misled by a demon by means of his senses, and as such those that are beyond doubt, given this scenario.
A final and I would say representative problem is raised by one of the replies, in which Descartes likens the process of doubt to tipping out a basket of apples, in the hope of discovering whether any are rotten. The trouble raised by this is that, within the metaphor, Descartes places himself outside the basket, which would seem to create another meta-metaphorical basket in his metaphorical head, which presumably would need tipping out also.
The fundamental problem appears to be that from the start, Descartes’ ideas about foundations are confused. Indeed, in later Meditations he begins to take foundational points apparently arbitrarily, through the introduction later on of “natural light” seemingly ex nihilo. Perhaps this is where the process of doubt might be improved.