Descartes claims that the Cogito Sum is indubitably certain beyond all reasonable and hyperbolic doubt.

Let us see, then, if there is a hyperbolic doubt which the purported truth of the Cogito Sum cannot overcome.

Dreamed persons are products of the human imagination.

Human beings pretend, or make believe, that dreamed persons can exist and think independently when, in fact, they cannot.

No dreamed person can exist or engage in cognitive activity independent of the imagination of the dreamer. This also means that the origin of a dreamed person and the origin of what a dreamed person may be doing or thinking about cannot be attributed to the dreamed person, per se, but must, instead, be attributed to the imagination of the dreamer.

Assume, further, that the dreamed person is performing the Cogito Sum in the first person, present tense mode, i.e., is performing the “When and while I am thinking, I must be existing.”

What can I conclude about this performance?

I can conclude, from the frame-of-reference of the dreamed person, that the truth of its Cogito Sum performance is indubitably certain because it is existentially consistent and existentially self-verifying.

However, since the dreamed person is unaware that its thinking and existing originate in and depend upon the dreamer’s imagination, the truth of the dreamed person’s Cogito Sum performance must be false.

Does this have any implications for the indubitable certainty of the truth of my own Cogito Sum performance?

I, too, conclude that the truth of my Cogito Sum performance, whenever I execute it in the first person, present tense mode, is indubitably certain because it is existentially consistent and existentially self-verifying.

But, like the dreamed person, it is possible that my thinking and existing may originate in the imagination of a dreamer of whom I am not aware. And since there is simply no way for me to prove definitively that this possibility is not the case, my Cogito Sum performance may be false, rather than indubitably certain.

In other words, this hyperbolic possibility constitutes a hyperbolic doubt that cannot be overcome by my Cogito Sum performance despite its existential consistency and existential self-verification.