Descartes said, "Cogito, ergo sum(I think, therefore I am)". I agree with him on the fact that this is the only thing that we can ever truly know for sure. Is there anyone who could tell me if there is anything else we could ever know for certain?

  • How about 1+1=2? Or are you looking for a different kind of possible knowledge?
    – E...
    Commented Oct 17, 2018 at 17:21
  • Isn't it impossible even to prove that logical knowledge such as "1+1=2" is true? It was Descartes who proposed the (hypothetical) Idea "What if some all powerful evil being was simply manipulating my thoughts." How could we ever prove this is not the case? Commented Oct 17, 2018 at 17:28
  • en.wikipedia.org/wiki/Evil_demon Commented Oct 17, 2018 at 17:31
  • "1+1=2" is not a logical law, but an arithmetical one. In order to prove it, we need arithmetical axioms. Commented Oct 17, 2018 at 18:23
  • It depends on one's standard of certainty, currently the question is too broad and vague to be on-topic here. In the rhetorically exalted sense that seems to be implied ("truly know for sure"?) cogito ergo sum isn't a certainty either, see Could 'cogito ergo sum' possibly be false?
    – Conifold
    Commented Oct 17, 2018 at 21:40

2 Answers 2


Descartes' famous Cogito is a very clever answer to his hyperbolic doubt, but it has had critics all along.

A very prominent critic is Bertrand Russell who claims in chapter II of his The Problems of Philosophy that the "I" in "I think, therefore I am" is not suffiently warranted. Descartes probably conceived of the Cogito in French, and he first wrote it in Latin, but both versions have the same vulnerability.

Russell's problem is that there is no certainty that the "I" remains identical to itself over time because it is known through experience and not a priori.

So, one can never be certain that the I that exits now is the same as the I that will exist, or did exist at any time other than now. Worse, the existence of the I is an inductive inference from various sense-data and is only probable. So, the Cogito, as stated cannot be known to be true a priori, as Descartes claims.

An earlier critic, David Hume, argues in An Enquiry Concerning Human Understanding (Sect. XII, Part III) that "The existence, therefore, of any being can only be proved by arguments from its cause or its effect; and these arguments are founded entirely on experience. If we reason a priori any thing may appear to produce any thing."

But these are empirical perspectives from which nothing can be known to a certainty. Descartes' own rationalist perspective seems replete with some necessary truths, such as that an effect cannot exceed its cause in any way because of the inviolability of Ex nihilo, nihil fit (a further assumed certainty).

However, Descartes' own device of the evil genius doubt seems entirely insurmountable. He claims that the evil genius could never bring it about that he is nothing when he thinks he is something, but that claim contradicts the definition of omnipotence.

  • Nice answer. The objections you mention (and others) may cause us to doubt Descartes' axiom but it will not cause us to doubt 'I Am'. If he'd left it at that there could be no objections. Even a genius could not cause us to think 'I Am' when we're not. As to whether the 'I';of Descartes exists. in religion the power that allows us to become muddled on this question is called Maya, and this natural phenomenon might be thought of as an evil genius in the manner of the Gnostics.
    – user20253
    Commented Oct 18, 2018 at 16:47

Well, the obvious, the death of your current physical (gross) body is the only guarantee in life.

I came across some relevant information, although it insists you believe that mathematics holds to a standard of certainty:

Immortality is Mathematically Impossible

Additionally, if you are familiar or have any interest in mathematics, I'd recommend checking out these mathematical phenomena below. You may certainly enjoy trying to rationalize them:

The Mandelbrot Set, which ties into "The Buddhabrot"

  • I know people who are convinced they have life everlasting. Given that they could conceivably be right, death isn't certain. As to the physical world, we can't be sure of that, since people demonstrably have false beliefs and delusions about it (we can't prove who, necessarily, but beliefs and claims differ), so we can't be sure of anything in the physical world (including its existence). Commented Oct 17, 2018 at 18:20
  • We only have inductive evidence that we'll die: so far everyone else has died. This makes it exceedingly likely that we will die, too, but not certain.
    – E...
    Commented Oct 17, 2018 at 19:20
  • Try not paying your tax for a while
    – Richard
    Commented Oct 17, 2018 at 19:55
  • @EliranH In fact, only about 93% of the people who've ever lived have died. That's insufficient to reject the hypothesis "not everybody dies" at the p<.05 level. Commented Oct 17, 2018 at 21:19
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    This does not seem to be an exhaustive, serious answer to the question as stated. This is what we are looking for, as you can see e.g. here. Please try to write answers that are longer, exhaustive, and ideally sourced (libraries could be filled with books on the question of "certain" knowledge/propositions alone). You can edit the existing answer to meet these requirements.
    – Philip Klöcking
    Commented Oct 17, 2018 at 21:35

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