Doubt is intrinsic to your constitution as a knower. It is given to you as a potentially bottomless power, but being corrosive and dangerous, you treat it with caution. The levels of experience, are the stabilisations of doubt which produce the various contracts between subject and object, in waking life or in dream or dreamless sleep. Experience is the abeyance of doubt, but to open to the doubt that still dwells at the core of experience is to touch on the certainty beyond that doubt. And if you go deep enough it annuls whatever relations between subject and object you have come to rest in. It is always possible to take it further as long as any kind of thought is possible. This is called inquiry, but it is the will to sacrifice everything at the altar of doubt. You could call it the higher Cartesianism.