Let's take this week for example. I know that I exist now on Monday, and that this coming Thursday won't begin all existence. Therefore Last Thursdayism for the future is disproved. This means that when I was born, my self evident existence disproved all future Last Thursdayisms for the rest of my life.

The counter argument would be everything could have popped into existence right now (or last Thursday), including my memories. But I know that this can't be the case for anytime in the future since I exist now, so if you were to tell me on Friday that I came into existence Thursday, it would have to be false.

I understand Last Thursdayism is a parody on creationism and isn't meant to be taken too seriously, but having said that, does my logic hold? This is strictly about Last Thursdayism and not about God or anything of that nature.

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    But hang on, what if it's only an evil demon making you think that today is Monday? It might already be next Thursday!
    – causative
    Feb 14, 2022 at 22:28
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    @armand There's no need, it's just for a fun though experiment. It's often said LTism is impossible to disprove so I'm wondering if my idea holds up.
    – user289980
    Feb 14, 2022 at 23:11
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    Everything is uncertain, because human thought processes are fallible. Last Thursdayism is possible. Two and two might actually add up to seven; haven't you made arithmetic errors before? And if you can make an arithmetic error once, isn't it possible you might make the same error a thousand times in a row? The question is never whether some absurd scenario is possible, because it always is. The question is, instead, what the balance of evidence says, while admitting this never yields perfect certainty.
    – causative
    Feb 14, 2022 at 23:21
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    Your argument is valid only against a claim made in the future. You can say "we are now monday, if someone tells me friday the wold was built thursday it will be false", but comes friday, when someone actually makes the claim to you, you wont be able to use your argument because they will argue your memories of monday could be fake. The rational counter argument against this kind of unfalsifiable hair splitting, solipsism, omphalos theory, variation in the speed of light, etc... is "what are your positive reasons to believe it is actually the case ?", "why should I believe like you do ?"
    – armand
    Feb 15, 2022 at 1:11
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    @causative what you say makes sense from a pure bayesian perspective, but you have to consider the economics of thoughts: thinking takes time and energy, and think about worthless hypothesis is a waste. Is last-thursdayism worth even the time for me to think "i assign an infinitesimal credence to this hypothesis" ? Certainly not, just like the myriads of other frivolous ideas without any evidence like the moon is made of cheese or that our dreams are produced by fairies living on its far side. I simply dont have time to entertain every single baseless idea, for they are infinite in number.
    – armand
    Feb 15, 2022 at 4:06

4 Answers 4


My current existence now disproves any future Thursdayism, but doesn't disprove any past Thursdayism. However, this statement is true in the future as well, so on Feb 18 (the next Friday at the time of writing), I will not be able to produce any data that disproves that the world was created on the 17th, the fact that I will have memories of writing this response now (on the 15th) notwithstanding.


Your argument basically boils down to "the universe wasn't (won't have been?) created next Thursday, therefore, it wasn't created last Thursday," which is a non sequitur, since conclusion is unrelated to the premise. Last Thursdayism claims that the universe came into existence last Thursday, the 10th of February, 2022. Your counter argument is saying that, on Friday, the 18th, you will be able to claim that the universe did not come into existence on the 17th, which is irrelevant to the "fact" that it came into existence on the 10th.


I know that I exist now on Monday, and that this coming Thursday won't begin all existence. Therefore Last Thursdayism for the future is disproved.

You are not disproving Last Thursdayism. This coming Thursday is irrelevant to Last Thursdayism. You are disproving a different claim.

This means that when I was born, my self evident existence disproved all future Last Thursdayisms for the rest of my life.

Possibly, but you don't actually know today that you existed at the time. So, if you had the proof at the time, you no longer have it today.

At any point in time that we are conscious, we know we exist, and this falsifies for us today any future Last Thursdayism claim. The fallacy, however, is to conclude from the fact that you know you exist now that you know that you existed before last Thursday.

The point of Last Thursdayism is that while we know we exist now, we can only believe that we existed in the past.

The fact that we believe today that we knew yesterday that we existed yesterday doesn't change the fact that today we can only believe that we existed yesterday.


The key premise in Last Thursdayism is memory doesn't guarantee that there is a past for it seems possible to, well, implant memories. False memory phenomena like the Mandela effect and others have been reported/documented.

Suppose today is Friday, 18/02/2022. All that you remember today, especially those parts that extend way back into your past, could be a memory implant and the world could've been created on Thursday, 17/02/2022

Come next Friday which is, according to my calendar 25/02/2022. How do you know that the world wasn't created on Thursday 24/02/2022? You would have to say I remember all the days between Friday 18/02/2022 till Friday 24/02/2022 i.e. you would have to depend on your memory. However, the reliability of memory as an accurate record of an actual past is exactly what was thrown into doubt. Put simply, your memory is useless and ergo you could never say, even if we consider the future, that Last Thursdayism is false.

That's my two bitcoins worth!

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