The following is excerpted from my "Does Existence Itself Exist? Transcendental Nihilism Meets the Paradigm Theory" in The Philosophy of Panayot Butchvarov: A Collegial Evaluation, ed. Larry Lee Blackman, The Edwin Mellen Press, 2005, pp. 57-73, excerpt pp. 67-68.
If anything can count as an established result in philosophy, it is the soundness of Descartes' famous cogito ergo sum 'argument.' Thus to the query, 'How do I know that I exist?', the Cartesian answer is that the very act of doubting that one exists proves that one indubitably exists. Now this may not amount to a proof that a substantial self, a res cogitans, exists; and this for the reason that one may doubt whether acts of thinking emanate from a metaphysical ego. But the cogito certainly does prove that something exists, even if this is only an act of thinking or a momentary bundle of acts of thinking. Thus I know with certainty that my present doubting is not a nonexistent object. But if Meinong were right, my present doubting could easily be a nonexistent object, indeed, a nonexistent object that actually has the property of being indubitably apparent to itself.
For on Meinongian principles, I could, for all I could claim to know, be a fictional character, one who cannot doubt his own existence. In that case, the inability to doubt one's own existence would not prove that one actually exists. This intolerable result certainly looks like a reductio ad absurdum of the Meinongian theory. If anything is clear, it is that I know, in the strictest sense of the word, that I am not a fictional character. My present doubting that I exist is an object that has the property of being indubitable, but cannot have this property without existing. It follows that there are objects whose actual possession of properties entails their existence. This implies the falsity of Meinong's principle of the independence of Sosein from Sein, and with it the view that existence is extrinsic to every object. Forced to choose between Descartes and Meinong, we ought to side with Descartes.