The claim is that if the experience isn't veridical then your belief is false and that if your experience is veridical then your belief is just lucky. So the sceptic doesn't need to claim that there are or aren't any trees. He just argues that either way there isn't knowledge. — Michael

But how is skeptic supposed to prove that if my experience is veridical then it is lucky? I'm not quite sure what you mean by "luck" here, but at least in the literature it means roughly something like basing your belief on an epistemic policy which doesn't reliably track the truth. But what makes a policy reliable is its relation to the environment in which is is exercised, so that means that you must know some facts about the world in order to assess whether my perceptually based beliefs are reliable. If my capacity to visually distinguish trees from non-trees (in the environment in which I in fact live) is reliable, then my beliefs which I form on the basis of experience cannot be lucky when true - and the mere possibility the this very same policy could misled me in some other worlds (in which there are no trees) is irrelevant to the question whether my believes are in fact lucky.

To be able to get beyond luck to actual knowledge you must somehow know that your experience is veridical, which according to the sceptic isn't possible. — Michael

It seems to me that you are changing the argument (of course you are welcome to formulate a different argument). Premise (1) in the argument as I formulated it, doesn't say that you must know that your experiences are veridical but something else. That would be a different argument.

or they can claim that they are defined as being the experience of an external and imaginary world respectively, in which case both types of experience fall under the umbrella term "dreaming" (even though they have other properties to distinguish them). — Michael

How does it follow? Surely being awake doesn't fall under the umbrella of "dreaming" (because it isn't dreaming).

But how is skeptic supposed to prove that if my experience is veridical then it is lucky? I'm not quite sure what you mean by "luck" here, but at least in the literature it means roughly something like basing your belief on an epistemic policy which doesn't reliably track the truth. But what makes a policy reliable is its relation to the environment in which is is exercised, so that means that you must know some facts about the world in order to assess whether my perceptually based beliefs are reliable. If my capacity to visually distinguish trees from non-trees (in the environment in which I in fact live) is reliable, then my beliefs which I form on the basis of experience cannot be lucky when true - and the mere possibility the this very same policy could misled me in some other worlds in which there are no trees is irrelevant to the question whether by believes are lucky. — Fafner

It's the example of the real painting and the forgery. I'm given one and believe that it's real. In the case that I'm given the real painting my belief is true, and in the case that I'm given the forgery my belief is false. But given that I can't distinguish between the real painting and the forgery, in the case that my belief is true I'm just lucky. It's a Gettier case.

And so too with the case of a veridical and a non-veridical experience.

How does it follow? Surely being awake doesn't fall under the umbrella of "dreaming" because it isn't... — Fafner

I said that if "dreaming" is defined as being of an imaginary world, and if the experiences which we claim to be waking experiences (defined as experiences of an external world) are actually of an imaginary world, then those experiences aren't actually waking experiences but dreams.

Sure, it's logically possible that life - by analogy - has the structure of a dream, but we really have no reason to suppose so (at least in our ordinary consciousness). Lacking a reason to suppose so means that doubting it would be irrational. — Agustino

However it seems quite possible that many people may be subject to delusions. That sure is evident from many of the contributions made here. So the possibility that one's understanding is fundamentally delusional or deeply mistaken about the nature of reality should always be considered. Indeed I think that was the original impulse behind sceptical arguments.

So the possibility that one's understanding is fundamentally delusional or deeply mistaken about the nature of reality should always be considered. Indeed I think that was the original impulse behind sceptical arguments. — Wayfarer

That makes little sense though. Otherwise we'd always be doubting ourselves, and we wouldn't be able to get anything done. Quite the contrary, if we don't have any reason for doubt, then we shouldn't doubt.

Otherwise we'd always be doubting ourselves, — Agustino

There is niggling, destructive scepticism - the kind of schoolyard 'prove it!' game that is intrinsically juvenile - but there's also a genuine questioning of what we normally take for granted or presume is beyond doubt. The latter is essential to philosophy.

But given that I can't distinguish between the real painting and the forgery, in the case that my belief is true I'm just lucky. — Michael

And what does it mean to being able to 'distinguish' between the two? I formulated the condition for knowledge in terms of having evidence that favors the one possibility over the other (and by 'favors' I meant having evidence which presence entails the truth of what is believed). But it seems to me that you have something else in mind here, and so as I said, this objection seems to me irrelevant to the argument that I'm considering.

(and good look formulating a deductively valid argument which derives from the assumption that one cannot distinguish (in some possible circumstances) between p and not-p, the conclusion that one cannot know that p - since the conclusion doesn't logically follow from this premise alone).

I said that if "dreaming" is defined as being of an imaginary world, and if the experiences which we claim to be waking experiences are actually of an imaginary world, then those experiences aren't actually waking experiences but dreams. — Michael

But claiming something to be a waking experience is a different matter from it actually being a waking experience, so what you are describing is not an example of a waking experience that "falls under the umbrella of "dreaming" ", but just a plain instance of dreaming.

But claiming something to be a waking experience is a different matter from it actually being a waking experience, so what you are describing is not an example of a waking experience that "falls under the umbrella of "dreaming" ", but just a plain instance of dreaming. — Fafner

You weren't reading properly. I'm saying that if the experiences which we refer to as waking experiences are of an imaginary world, and if waking experiences are defined as experiences of an external world and dreams defined as experiences of an imaginary world, then those experiences are dreams, not waking experiences.

because what does it mean to be awake if not to be in a state which is logically inconsistent with having a dream? — Fafner

Being in a state that is logically inconsistent with being asleep and dreaming does not imply that one can distinguish the two states, because the inconsistency does not stem from the difference in the mental state but from the fact whether one is awake or asleep.

And what does it mean to being able to 'distinguish' between the two? — Fafner

To be able to determine if I'm having a veridical experience or a non-veridical experience. If I can't determine which I'm having then I can't know which I'm having.

To be able to determine if I'm having a veridical experience or a non-veridical experience. — Michael

And what does it mean to "determine"?

And what does it mean to "determine"? — Fafner

To recognise some feature that veridical experiences have and non-veridical experiences don't (or vice versa).

Being in a state that is logically inconsistent with being asleep and dreaming does not imply that one can distinguish the two states, because the inconsistency does not stem from the difference in the mental state but from the fact whether one is awake or asleep. — BlueBanana

I didn't say anything about being able to distinguish between the two, I only said that if you are awake then you have perceptual evidence which rules out the possibility that you are dreaming (which if true, proves that premise (3) in the argument is false).

To recognise some feature that veridical experiences have and non-veridical experiences don't. — Michael

And how does it logically prove that I don't know that I'm having a veridical experience from the fact that I can't recognize such a feature?

The perceptual evidence, via which you'd be able to distinguish between the two. I'll rephrase my comment:

Being in a state that is logically inconsistent with being asleep and dreaming does not imply there is perceptual evidence of being in any state, because the inconsistency does not stem from the difference in the mental state or perceived reality but from the fact whether one is awake or asleep. — BlueBanana

I'll also quote another part of your post:

But if this is the case, then contra the skeptic, being in the one state as opposed to the other does after all entail information about how the external world actually is, because the way things appear to you when you are awake usually matches very closely the way they really are – something which is plainly not the case (at least most of the time) when one is dreaming. — Fafner

There is a hypothetical scenario where the other state does not entail that information, of which you'd be completely unaware of because it's your only source of how the things are.

And how does it logically prove that I don't know that I'm having a veridical experience from the fact that I can't recognize such a feature? — Fafner

If you can't recognize it, how do you claim you know it?

Being in a state that is logically inconsistent with being asleep and dreaming does not imply there is perceptual evidence of being in any state, — BlueBanana

What do you mean by "perceptual evidence"? According to how I understand evidence, something is evidence for p, if its presence entails the existence of p. Now if having a waking experience defined as a state such that things necessarily match the way they appear to me, then if I'm having a waking experience I'm in a state which entails the presence of whatever that I perceive - and this is just a case of having evidence according to my definition.

There is a hypothetical scenario where the other state does not entail that information, of which you'd be completely unaware of because it's your only source of how the things are. — BlueBanana

But this is irrelevant to the question whether I can know how things are. The mere possibility of some state in which I would be dreaming that I'm awake, doesn't prove that I cannot know anything when I am awake.

And how does it logically prove that I don't know that I'm having a veridical experience from the fact that I can't recognize such a feature? — Fafner

If you can't recognize it, how do you claim you know it? — BlueBanana

Perhaps because the possibility is irrelevant to my knowledge. There's no reason to assume that whenever I know something, then I must be able to recognize all possible cases of deception or illusion. Perhaps I need to be able to recognize only some.

And how does it logically prove that I don't know that I'm having a veridical experience from the fact that I can't recognize such features? — Fafner

From your first premise: For any p, if a subject doesn’t have evidence which rule out (perhaps conclusively) the possibility of an error regarding p (that is, favor the possibility that p over not-p), then the subject doesn’t know that p.

If you can't recognize a feature that veridical experiences have and non-veridical experiences don't (or vice versa) then you don't have evidence which rules out the possibility of an error regarding your belief that your experiences are veridical, and so you don't know that your experiences are veridical.

Just as if I can't recognise a feature that the real painting has and the forgery doesn't (or vice versa) then I don't know that it's a real painting.

But the first premise doesn't say that in order to know p you have to recognize states of having evidence for p according to some distinguishing feature, it merely says that it's enough if you just have the right sort of evidence, but doesn't mention recognition. You have to add a further premise here to make the argument valid.

You seem to be conflating. I'll set it out more clearly.

1. We have experiences of type A and experiences of type B.
2. We refer to experiences of type A as "wakefulness" and experiences of type B as "dreaming".
3. We claim that wakefulness is the experience of an external world and that dreaming is the experience of an imaginary world.

There are two ways for the sceptic to approach this. They can either claim that "wakefulness" and "dreaming" are defined by their referents, in which case our claim that wakefulness is the experience of an external world is false, or they can claim that they are defined as being the experience of an external and imaginary world respectively, in which case both types of experience fall under the umbrella term "dreaming" (even though they have other properties to distinguish them).

Nothing about this "destroys meaning" or "short circuits" language.

But again, this is just pedantry. The sceptic's claim is simply that we can't know that our experiences are of an external world, regardless of what we call them or think of them. You can't counter this by pointing to a dictionary. — Michael

I don't think I'm conflating anything.

If experiences of type A are actually "dreaming" as well, then what grounds our concept of wakefulness? :s We have no experiences left to ground it, because even type A experiences are dreaming. But without grounding wakefulness (the real world), how can we ground the imaginary? Without the reference of the real, the imaginary cannot be imaginary.

Similarly. If type A isn't external world and is also internal, then what experience grounds our concept of external? And if none, how can we then talk of internal? Internal only exists with reference to what is external. By undermining one side of the dichotomy, the skeptic effectively destroys the meaning of the terms.

it merely says that it's enough if you just have the right sort of evidence, but doesn't mention recognition. You have to add a further premise here. — Fafner

But simply having a veridical experience isn't evidence that it's a veridical experience, just as simply having the real painting isn't evidence that it's the real painting. It's evidence only if it allows you to rule out the possibility that you're wrong, and for that you need to recognise some distinguishing feature.

I don't think I'm conflating anything.

If experiences of type A are actually "dreaming" as well, then what grounds our concept of wakefulness? — Agustino

What grounds our concept of Gods, or demons, or unicorns, or ghosts, and so on? We don't need to have had an experience of an external world to have the concept of an experience of an external world.

of Gods — Michael

Experience.

or demons — Michael

Experience.

unicorns — Michael

Experience (we experienced both horns and horses).

We don't need to have had an experience of an external world to have the concept of an experience of an external world. — Michael

Yes we do need to have had an experience of exteriority - something external to us - in order to ground the concept of external world.

Yes we do need to have had an experience of exteriority - something external to us - in order to ground the concept of external world — Agustino

So I must have seen a ghost because I have the concept of ghosts? That's wrong. I have the concept, but I've never seen one.

A brain in a vat can conceive of an external world.

So I must have seen a ghost because I have the concept of ghosts? That's wrong. I have the concept, but I've never seen one. — Michael

What's your concept of a ghost? The concept of a ghost is a composed concept - composed of multiple atomic concepts, just like unicorns. Its atomic concepts, you have had an experience of all of them.

What's your concept of a ghost? — Agustino

A disembodied person.

Its atomic concepts, you have had an experience of all of them.

So I've had the experience of a disembodied thing? I haven't.

Again, brains in a vat can conceive of an external world.

Furthermore, at best your argument can only have you conclude that at some point you've had an external world experience. That doesn't help you to determine if the experiences you're having now are of an external world.

Maybe right now you're a brain in a vat, and your concept of an external world is a relic of your pre-envatted life.

So I've had the experience of a disembodied thing? — Michael

Yes. By disembodied you refer to things like light - light has no body (according to your own beliefs in the other thread). Think about what you imagine when you imagine a ghost.

Furthermore, at best your argument can only have you conclude that at some point you've had an external world experience. — Michael

Then you'd need to specify that point and also why you think your experience was of an external world then, and not now.

Yes. By disembodied you refer to things like light - light has no body (according to your own beliefs in the other thread). Think about what you imagine when you imagine a ghost. — Agustino

Except I don't imagine ghosts to be electromagnetic radiation. I imagine them to be non-physical things.

Then you'd need to specify that point and also why you think your experience was of an external world then, and not now.

No I don't. All I need to do is accept that at some point I've had an external world experience to explain the origin of such a concept (assuming I accept your premise, which I don't). This doesn't undermine the sceptic's claim that we can't know that the experiences we now have are of that external world.