Certainty, Necessity, Objectivity

Rationalist Skepticism targets Sense-Experience; Empiricist Skepticism targets the connection between Sense-Data.  Hume specifically challenges the attribution of Necessity to the connection between Cause and Effect, on the grounds that the attribution is based on merely an observed repeated conjunction, a challenge with which Descartes might agree.  Kant's response to Hume is based on a distinction between Objectivity and Subjectivity that Hume, like Descartes, does not seem to recognize, i. e. that 'A causes B' entails that B could not have been perceived prior to A, unlike in the case of a merely subjective sequence of perceptions.  Kant thus recasts the Certain-Doubtable and Necessary-Contingent distinctions as Objective vs. Subjective, with the implication that Skepticism, whether Rationalist or Empiricist, as a Subjective condition, has no Objective relevance.  Now, Objectivity connotes independence from Subjectivity.  So, if Descartes' 'I exist' is Objective, then it is independent of his thinking that it does.  But, he never establishes the independence of Sum from Cogito, or that of Cogito from any mental operation.  Likewise, the only Certainty that he establishes is that of 'If I think, then I exist', not that of 'I exist'.  Hence, either his Method establishes that there is no Certain existence, or it is inherently flawed, i. e. because it lacks the capacity to succeed as a means of establishing that there is an existence that is Certain.