What follows is a revision of the position I took in a previous Discussion posted on The Philosophy Forum entitled: Why Descartes’ Cogito Sum is Not Indubitably Certain.

Thanks to those whose respectful, critically constructive comments prompted this revision.

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The Cogito Sum performance of a fictitious person in my dream is totally dependent on my imagination, i.e., the fictitious person thinks only because I am imagining it, the fictitious person exists only because I am imagining it, and the fictitious person performs the Cogito Sum only because I am imagining it.

From my frame-of-reference as dreamer, the fictitious dreamed person owns neither its thinking, its existing, nor its Cogito Sum performance because the dreamed person is not their original source. I am.

The dreamed person can perform the Cogito Sum, in the first person, present tense mode, but the dreamed person’s thinking (Cogito) and existing (Sum) do not have their original source in the dreamed person and do not belong to the dreamed person - they belong to me.

If the dreamed person’s thinking and existing are totally dependent on me, the dreamer, as their original source, this would not necessarily undermine the indubitable certainty (the existential consistency and the existential self-verification) of the dreamed person’s Cogito Sum performance, in the first person, present tense mode, but it certainly would undermine the authenticity of the dreamed person's Cogito Sum performance.

From the dreamer’s (my) frame-of-reference the dreamed person would be engaging in what is admittedly an indubitably certain, yet completely “ersatz,” Cogito Sum performance.

The complete dependence of the dreamed person’s thinking and existing on the dreamer’s (my) imagination, along with the essentially “borrowed” nature of both, would not jeopardize the indubitable certainty (i.e., the existential consistency and existential self-verification) of the dreamed person’s performance claim in the first person present tense mode: “When and while I am thinking, I must be existing,” but it certainly would jeopardize the authenticity of that performance claim.

The dreamer’s (my) imagination is the original source of the dreamed person’s Cogito (thinking), the original source of the dreamed person’s Sum (existing), and, therefore, the original source of the indispensable constituents of the dreamed person’s Cogito Sum performance.

For the dreamed person's Cogito Sum performance to be authentic, one would have to presuppose that the performance also yields an equally indubitably certain intuition, in the first person present tense mode, that the dreamed person is always the sole source of the constituents of its Cogito Sum performance - which, in this case, it clearly does not.

I submit that the foregoing circumstances create a hyperbolic doubt which makes it possible to reject the authenticity, but not the indubitable certainty, of the dreamed person's Cogito Sum performance.

But then the question arises: Might it be possible for me (the dreamer), when awake, to perform, in the first person present tense mode, an existentially consistent and existentially self-verifying Cogito Sum which would confirm, with indubitable certainty, that I am also the sole originating source of the constituents of my Cogito Sum performance?

I submit the answer to this question would still be NO!!!

When I am awake, the truth of my Cogito Sum performance will be indubitably certain, but, even then, it will be an inauthentic performance, because it will be impossible for me to ascertain, with indubitable certainty, that I am the sole originating source of the constituents of my performance.

My situation, in this regard, will be no different than that of the fictitious person of whom I dreamed.

Furthermore, it is interesting to note that even the Evil Genius postulated by Descartes did not have the power to bring into question the indubitable certainty of Descartes' Cogito Sum performance, but the Evil Genius, like the dreamer, did have the power to bring into question the authenticity of Descartes' performance.