Ariadne ,
An analysis implies an examination of an element of the "whole". But if the "whole "
was to be understood ,there would not be any need for an analysis.
Perhaps Descartes did not see the "whole ".
Obviously he can not believe his own senses ; he functions in knowledge ,past , thought ,memory .On the other hand senses are an awareness and always in the present ; from moment to moment.(I smell now , I feel now , I see now etc) .
Can one be aware of one self ?.....only when one discards the past , the dead .
Knowledge, belief, and truth are not related to wholes unless you want to
sell religion (any denomination). The
sum of the parts being greater than the whole argument is not related to Descartes ... he subtracts everything, arrives at zero, and then begins to rebuild ... all the way to God.
It's not obvious that Descartes cannot believe his senses. He says that he sees/hears/tastes/feels/smells/senses something, but he cannot be sure that what he senses is real or a figment of his imagination. He cannot deductively prove that it is real, other than what he perceives through his senses (which may be deceptive), so he assumes that his senses, one by one, are false - or cannot be verified/validated. Eventually he argues that the only thing he can verify (knowledge) is his own existence (truth). The exercise was not intended to prove his existence (belief). Many west coast philosophers liked his ideology and built on it. Bertram Russell came later, but the origin of the argument is the same.
To fully appreciate his argument about knowledge, belief, and truth, you have to read his discussions.