Method, Certainty, Infallibility

Certainty can be either Theoretical or Practical--the former is Necessity of Truth, while the latter is Infallibility.  Thus, for example, a Proposition that is certain is necessarily true, while a Method that is certain is one in which success is guaranteed.  Likewise, to doubt a Proposition is to consider that it might be false, while to doubt a Method is to consider that it might be unsuccessful.  Now, a notable confusion of the two varieties is in a traditional argument against Skepticism--that applied to itself it cancels itself--the pioneering version of which is Descartes'.  But Skepticism is in this case a Method for determining Theoretical Certainty.  Hence, it is inapplicable to itself, which is why Descartes cannot doubt that he is doubting, and, thus the presumed contradiction--that he is both doubting and doubting that he is doubting--does not obtain.  Likewise, because the negation of Practical Uncertainty is not Theoretical Certainty, as a Method of determining Theoretical Certainty, his Skepticism about Skepticism fails.