The Myth of Absolute Certainty

I was banned from Uncommon Descent this morning for reasons unknown (though here is a plausible hypothesis). At the time of my banning, I was in the midst of a long discussion of absolute certainty and whether it can rationally be claimed. Since I can’t continue the discussion at UD, I’ll start a thread here instead and solicit the opinions of the very smart locals here at TSZ.

The question is whether there we can be absolutely certain of anything. I am not speaking of absolute certainty in the colloquial sense (“I’m absolutely certain I left the keys on the counter!”), but in the precise sense of 100.0% (unrounded) certainty, with literally no possibility at all of error — not even a trillionth of a trillionth of a trillionth of a percent chance of error.

It seems obvious to me that we cannot rationally claim that kind of certainty because we know that our minds are fallible. We know that we can be mistaken without realizing it, even in cases where we feel absolutely certain. The example I used at UD was the certainty many 19th-century scientists felt, before Einstein, about the “self-evident” absolute and distinct nature of space and time.

Given the ever-present possibility of error, I think it’s pure hubris to claim absolute certainty of anything – even something as seemingly inescapable as Descartes’ famous cogito.

Not surprisingly, the regulars at UD disagreed.  Kairosfocus in particular was in hysterics over the supposed dire consequences of my view. What surprised me, though, was that Lizzie also disagreed with me. I am interested in hearing more about why she disagrees, and also in what the rest of you think.

Is absolute certainty possible?  If so, what can we be absolutely certain about?  What (if anything) justifies absolute certainty?  I look forward to your answers.

109 thoughts on “The Myth of Absolute Certainty”

  1. PatrickPatrick

    Alan Fox:
    ETA: It occurs to me that mentally putting Los Angeles in a plastic sack is equally futile

    But so potentially beneficial that it’s worth some effort!

  2. keithskeiths Post author

    kairosfocus, at UD:

    PPS: Implication logic [where warrant for implication has various grounds] is different from entailment of sets of syllogisms, which are about combined assertions regarding set membership, directly implicating meaning. For instance P: Socrates is a Man plus Q: men are mortal entails that R: Socrates is mortal. Interpreted on the implications side, it brings up that one or both of the members of {(P AND Q) => R}, which per De Morgan etc or a truth table, converts to ([P => R] OR [Q => R]). I was puzzled on this as a student and went looking for a Mathematician in that Dept at my Uni. The point was underscored that the meaning involved in each of P and Q does have that property. Socrates is a man involves the constitution of being human, i.e. mortality. That may help clarify a difference that seems to be under debate.


    Either you misunderstood the professor, or he gave you a bad answer.

    You are interpreting “X -> Y” as meaning “if X is true, then Y is true”. That’s what we mean when we say “X implies Y” informally.

    In formal logic, however, “X -> Y” is a statement like any other. That means it can be either true or false, depending on the values of X and Y, and X and Y can take on any combination of values.

    X and Y also need not have anything to do with each other. Regardless of what X and Y stand for, the value of “X -> Y” can be obtained from the appropriate truth table.

    For example, let X stand for “John is a Republican” and let Y stand for “Salmon swim upstream to spawn.” Suppose John really is a Republican, so that X is true. We already know that salmon swim upstream to spawn, so Y is true. Looking at the truth table, we see that if X and Y are both true, then X -> Y is also true. However, that obviously does not mean that John’s party affiliation determines whether salmon swim upstream to spawn.

    If you keep this in mind, your confusion over the Socrates syllogism will vanish.

  3. r0bb


    Thank you for your post, Stephen. It is a perfect illustration of why absolute certainty — particularly in the hands of sloppy thinkers like you — is a dreadful mistake.

    We should give Stephen credit for finally recognizing a long-standing error of his. From his OP:

    if one begins with a false premise and reasons perfectly, he will arrive at a false conclusion.

    He responded to counterexamples by claiming that this applies only if there is a single premise, apparently not realizing that any set of premises A, B, C, … can be restated as a single premise “A and B and C …”, which is false if any of its components is false.
    But he later conceded that his belief was mistaken, to which I say Hallelujah.

    For years he has failed to recognize the unidirectionality of implication, thinking that a true consequent can only follow from a true antecedent. For example:

    Sufficient causes:
    If x is a sufficient cause of y, then the presence of x necessarily implies the presence of y.

    Because of those definitions, if one uses the phrase, “if an event occurs,” it automatically means that the sufficient cause[s] was [were] present.

    And he has long tried to demonstrate the truth of his claims by pointing to a true consequent of each claim. For example:

    Logic: A thing cannot be and not be at the same time.

    Validation: Pluto cannot exist and not exist at the same time. Get a life.

    Logic: A physical event cannot occur without a cause.

    Validation: The post you just wrote required an author. Get a life.

    Now that StephenB seems to have figured out that affirming the consequent is a fallacy, perhaps he’ll rethink some of his previous arguments, along with his condescension toward those whom he lectures about “reason”.

  4. keithskeiths Post author

    If we could arrange for StephenB to bet life or limb on it, I think we’d quickly discover that StephenB is not absolutely certain of his absolute certainties.

  5. OMagain

    If we could arrange for StephenB to bet life or limb on it, I think we’d quickly discover that StephenB is not absolutely certain of his absolute certainties.

    Or, indeed, a bottle of whiskey….

  6. Blas

    And still all the people that posts here beleive the fact that all the life beeing descent from a FUCA.

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