Let's look again at Descartes' cogito argument:
“let him [the deceiving demon] deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something. So that after considering everything enough and more, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.” (Second Meditation, 25)
In the replies to the objections to the Meditations, Descartes writes that this statement should not be understood as a logical argument with a tacit premise "Whatever thinks, is or exists" but something to be recognized as known directly by a "simple intuition of the mind".
Still this line of reasoning starts with the claim that we can surely know to be conscious and thinking.
Indeed, it seems impossible to doubt that I am conscious, since I have conscious experience, now, at the same time t, while I do the doubting.
But I surely can legitimately doubt that I was conscious at any time t - Δt in the past from now, t.
Time will pass and at the moment t + Δt, the fact, that I was conscious at time t cannot be immediately experienced by me, anymore. "I was conscious at time t" is just a supposed 'fact' stored in my memory, which everybody (including Descartes) agrees, can be doubted.
Inductively, "It is doubtful, that I was conscious at time t" is true for any t, so it seems that the claim "I am conscious now" is a proposition that can be legitimately doubted.
Descartes didn't feel the need to explain that this reasoning is wrong. Why? Is this a genuine weakness in the argument, he didn't notice? Even if it can similarly resolved like "Aristotle's sea-battle", at least it questions that the cogito can be known as true by "simple intuition of the mind".