Novice here, so please don't use jargon or advanced topics (I've had experiences like this on the math stack exchange lol). I was reading through the book Philosophy for Everyone, and I was just reading through the radical skepticism section in the knowledge chapter. The book says that the "radical skeptic" claims that we don't know whether we are brains in vats, so we don't really know anything. But does this mean that we know that we don't know whether we are in a skeptical hypothesis scenario? Is this correct, or is there a subtle flaw with this? In other words:

Do we know that we don't know everything?

This would contradict the fact that we know nothing.

Sorry if my question isn't worded clearly (or if there's something completely wrong lol), again, I'm only a beginner.

  • What do you mean by the word "knowledge"? Do you know what you mean by that word? How does the assertive skeptic (not the "quiet" one) know that it is possible that we are brains in vats? Do you know that you will get an adequate answer to such questions without having to delve into the topic much more in-depth than you are requesting our answers to be? Oct 12, 2023 at 0:16
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    Distinguishing first order knowledge from second order (knowing that we know) is a standard practice against skepticism nowadays.
    – Hokon
    Oct 12, 2023 at 1:11
  • Compared to pyrrhonism, radical skepticism is self-refuting as old as the famous Epimenides paradox (Epimenides the Cretan says, 'that all the Cretans are liars,'). Even solipsism at least claims one really knows something substantial. Though it does need some jargon of advanced topics such as S4 epistemic modal logic to get to the bottom of your question: if we don't really know something, does this mean that we know that we don't know this thing on a higher order. Oct 12, 2023 at 6:10
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    From Wikipedia: "Some philosophical skeptics have responded to this objection by restricting the denial of knowledge to certain fields without denying the existence of knowledge in general. Another defense consists in understanding philosophical skepticism not as a theory but as a tool or a methodology. In this case, it may be used fruitfully to reject and improve philosophical systems despite its shortcomings as a theory." Side note: you can be unsure if you're a brain in a vat without claiming you can't know anything (i.e. solipsism).
    – NotThatGuy
    Oct 12, 2023 at 8:23

2 Answers 2


I understand your question perfectly fine. Your question would eventually lead strangely enough a solipsistic point of view where everything can be doubted apart from one’s own existence (Descartes “I think therefore I am”)

The same logic of doubt and scepticism can pretty much be applied to anything about the external world and it’s called Idealism where the external world doesn’t really exist but that it’s only consciousness or ideas. Brain in a vat is very similar to the idea that we’re living in a simulation by introducing doubt about what reality actually is, whether the external world is rendered by super powerful computers rather than being a fundamental aspect of reality.


There are several responses to radical skepticism depending on what premises are used to limit our knowledge of being a brain in a vat (BIV). There is also a strategy where we actually re-define knowledge based on context.

Direct objections like Putnam's are based on the impossibility of knowing about true BIVs if you are a BIV. See link below. https://plato.stanford.edu/entries/skepticism-content-externalism

The other option is to say that truth is contextual to: in the context of BIV, you can't know you have hands, in the context of "give me a high five", you can claim to know of and use your hands. https://plato.stanford.edu/entries/contextualism-epistemology/

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