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".

  • We have so many cogito threads, perhaps it deserves a tag. There are so many weaknesses in cogito, philosophy.stackexchange.com/questions/10003/… or philosophy.stackexchange.com/questions/70/… that there is no need to bring in the extraneous notions of time and induction, which are derivative abstractions unlike the "immediate intuitions" Descartes tries to capture, and hence thrown out by doubt at that point. In any case, he writes "true whenever it is", so other times are moot.
    – Conifold
    Jul 15, 2016 at 2:21
  • @Conifold: sure the cogito argument has flaws, but those are usually seen in the deduction. This question tries to discover if even the basic premise is flawed.
    – viuser
    Jul 15, 2016 at 9:57
  • If I am, who cares whether I was? The argument does not claim to prove continuity with the past, or the future. As Evangelicals commonly whip out to excuse nonsense, the Boltzmann Brain people are not crazy: God could have just created the universe but given you the false impression you had a past. The argument itself would still hold. At the instant you are thinking, you think, and so you exist.
    – user9166
    Jul 16, 2016 at 19:46
  • @jobermark: That's not the gist of the argument. The question was, if a proposition P is doubtful at all times but a knife-edge instant t might we not suspect, that it was doubtful at t, too?
    – viuser
    Jul 18, 2016 at 13:12
  • That is just Zeno's paradox. It is impossible for the Achilles to catch up with the tortoise at any time before he does, and then it happens. After that, they are never together either. Therefore it is dubious that they were ever tied? So unless you want to rule out motion, you need to discount this argument as a general form. Isolated points of exception exist, and general uniformily is not an argument against them.
    – user9166
    Jul 18, 2016 at 15:10

2 Answers 2


Your argument is not consistent. If you state that it is impossible to doubt that you are conscious at t, and later find it is reasonable to doubt that you are conscious at t, then either your logic is flawed or the assumptions are inconsistent.

In this case, I think the issue is the transition from exploring states t - Δt and t + Δt and using those to make statements about the state at t. In reality, the statement "But I surely can legitimately doubt that I was conscious at any time t - Δt in the past from now, t." is only valid for Δt>0.

Beyond that, there's plenty of skepticism to be had, but there is no straight forward mathematical inconsistency obliged in the argument.


I don't think your objection works for at least two reasons.

First, your objection hinges on an induction. Induction falls within the list of things we doubt.

Second and more importantly, there's a problem related to the temporality of logic and transcendental arguments at work here.

Following McTaggart's distinction, we could say Descartes is using an "A-Theory" approach in his argument.

 Conclusion: During the "now", an I qua thinking thing exists.
 Either it exists as it seems to  OR it exists as the thing doubted.

For this argument, Descartes can hold all other things as dubitable (as he eloquently writes at the beginning of Med I and again at Med II). In other words, while he's doing so, he can continue to doubt his memory, etc. the situation.

Your objection conversely seems to require a B-theory approach to the situation. There's no specific and privileged now, there's just indexed moments in time which constitute an indifferent dubitable series.

But there's no reason Descartes can't accept (for the sake of his argument) that all accounts / recollections of time, memory, and the existence of anything at any given time t are dubitable. In fact, it seems like he does. What he then does is conjoin this a privileged notion of the now such that in the "now" -- not before or after, there's an indubitable claimant to consciousness.

I think what you're seeing in the replies to objections is that he's not trying to make the argument static ("proposition" is a word with a specific meaning in the scholastic vocabulary Descartes learned rather than what we learn now based on 19th and 20th century logic and philosophy of language).

More broadly, the objection isn't too important, because Descartes is not a radical skeptic. Instead, he winds up being committed to just about all the normal things we believe but on the foundation of what Cottingham calls the Cartesian Circle (proof for the self that depends on proof for God; proof for God that depends on proof for the self; plus the magic of clear and distinct ideas).

So the memory of being a conscious being is dubitable

  1. in the broad sense that it can be doubted (unlike consciousness)
  2. in the narrower sense that as one of the things doubtable in the broad sense, it's excluded as a source of knowledge during the bracketing of the Meditations

But that doesn't mean that once we're outside the brackets that Descartes thinks it's particularly dubious (like sense perception), it's just not able to justify itself.

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