As I see it, if (1)(b) is logically possible, then (*) is coherent. That it seems to the subject that he's in state (a) (and (b) seems impossible to him) doesn't imply that he's in state (a). Which leaves (b) as a logical possibility. — Andrew M

Notice that I don't wish to deny the possibility to be in state (1b), of course that is a logical possibility. What I do deny is the possibility that one could be in the very same mental state as in (1a) and be deceived.

The skeptic of course doesn't claim to know whether I'm in state (1a) or (1b), but he claims that even if I'm lucky and I'm in fact in state (1a), the possibility of a mistake still exists, which I cannot rule out. But my point is that if one is in fact in state (1a) then there's no possibility of him to have the same experience and be mistaken.

Not had you eternally been in a dream, and had no sense of doubt due to everything in the world being the same as your level of knowledge about it. — Posty McPostface

But I don't see why does it have to be so.

And as I said before, even if we grant that you are correct, don't you agree that if an evil demon is causing your sensations (from outside your consciousness), you would then be able to doubt the evil demon world? (or imagine the scenario where the world has been completely destroyed 5 minutes ago, and only your brain is kept alive by the evil demon without you noticing anything. Surely in THIS world you can doubt both possibilities.)

In a dream everything is perfectly clear, there's no room for/to doubt the existence of the dream world itself because there is no room for doubt itself. — Posty McPostface

I think it's perfectly possible to ask yourself from within your own dream whether you are dreaming or not. Some people can in fact do just that (lucid dreamers).

Sorry I'm not following...

And of course the evil demon is merely an hypothesis, the skeptic doesn't actually claim to be able to prove that there is an evil demon (if this is what you meant).

But again, if the skeptic is only a local and not a global one, he can argue, without inconsistency, that some things cannot be doubted, while retaining much of his skepticism about everything else.

Therefore, if we take the evil demon path of assuming that everything can be doubted and nothing can be certain — Posty McPostface

Once again, the skeptic can restrict the scope of his skeptical claim and exclude the possibility of doubting one's doubt. The most interesting type of skepticism targets only the beliefs the could be doubted, but he is happy to exclude from his skepticism beliefs that logically cannot be doubted (such as the existence of the self, and perhaps some other things). But as it happens, the vast majority of our beliefs are such that they sensibly can be doubted, and thus if the skeptic is right then we are still in a serious epistemic trouble after all.

Well, not quite. Because since the world and the self are the same thing in solipsism, then there can be no room for doubting the world since the self must exist. — Posty McPostface

It seems to me that even if this inference is correct, it still would not refute skepticism because the skeptic need not argue for strict solipsism (in your sense) for his argument to be disastrous for knowledge. A world in which only I exits and the evil demon deceiving me into believing that there's an external world is just as bad as a strictly solipsistic world, but since it is not solipsistic in your sense, your argument is incapable of disproving its possibility. (because surely, in the Evil Demon world you CAN sensibly entertain (and hence doubt) both the hypothesis that the external world is real, and the possibility that you are being deceived by the demon.)

Well, if doubt can only exist when there is lack of certainty, and if one can doubt whether solipsism is an actuality, then at most it follows that solipsism is not certain. But I can't see how that proves anything about the existence of my hands.

Anyway, it seems to me like a weird strategy to affirm solipsism in order to refute skepticism, since the solipsist goes even further than the skeptic. And furthermore, solipsism is equally questionable as realism according to the skeptic, since you can't know neither the truth of solipsism nor realism.

Nice to see you too @Posty McPostface, thanks =)

concerning your question, I'm not sure what you mean by "epistemological solipsism", could you elaborate?

I think the argument would still work even if you replace 'know for sure' with a simple 'know'. According to the skeptical argument, all possible evidence for any proposition cannot support the proposition any better than its negation. If every experience of the external world you could ever have, can potentially turn out to be mistaken, than what reasons (even weaker than perfect certainty) could you possibly produce in favor of believing that you are not radically mistaken about the external world?

I do not wish do deny any possibilities. But what is crucial is HOW we describe those possibilities. If we agree that (a) and (b) are logically exhaustive, then the skeptic cannot, on pain of contradicting himself, maintain that whenever (a) or (b) is actually the case, I could be mistaken either way, because--by definition--I'm not mistaken if (a) is the case. So unless the skeptic can prove that (b) is actually the case, he is not entitled to premise (2).

Now, I grant the assumption that (a) and (b) are subjectivity indistinguishable mental states, but the crucial point is that they are DIFFERENT kinds of mental states nonetheless. The former is a state of seeing truly how things are in the world, while the latter isn't.

Thus, the realist cannot assume that (2b) is true, where (2b) is that it is always logically possible that you are actually seeing your hands and are not deceived. — darthbarracuda

I'm not sure what you mean here by "always logically possible". Surely if it seems to me that I see my hands, then it IS logically possible that I see my hands, if I in fact have them and am not deceived. Do you reject premise (1)?

What is wrong with the skeptical argument is that if premise (1) is true, then it's simply incoherent for the skeptic to maintain that I could be mistaken whether (a) or (b) is the case, since obviously if scenario (a) is the case, then I'm NOT mistaken. In other words, when the skeptic says "it's always possible that one is mistaken", it's not clear what he means by 'possible'. For if I do see (truly) that I have hands, then it is logically impossible for me to be in this very same state and be mistaken (that is, in the state of truly seeing that I have hands).

We have the same position here, i meant empirical verification only in the internal sense of methodological solipsism - as opposed to epistemological solipsism. In other words what is not cognizable in terms of first-person experiential phenomena is judged to be meaningless and lacking truth-value as opposed to being transcendentally right or wrong but unknowable. — sime

This cannot be right, because obviously not everything intelligible (having truth conditions) can be experienced from a first-person perspective, e.g., the past, elementary particles, very distant regions of space and so on. Sense experience is of course important, but I didn't say that everything should be defined relative to experience. I spoke of cognition and knowing in much broader terms. We shouldn't repeat the mistakes of the positivsts. And this is why I said that the idea of 'limits' to thought should not be thought of as trying to exclude something, or putting a-priori conditions on what does and doesn't make sense. To paraphrase Wittgenstein, the limits of language show themselves in language, but cannot be expressed or described by propositions. It is not a piece of knowledge that 'thought has limits', no more then 'p or not-p' is a piece of knowledge about something.

Unfortunately "mistakes" and "knowledge" in ordinary language are usually interpreted in terms of Truth-By-Correspondence, and this commonly held background assumption in conjunction with your "if" clause makes your paragraph read as if you at least concede to the dream-sceptic that the dream/reality distinction is logically conceivable in terms of T-B-C. — sime

What's 'T-B-C'?

There's nothing wrong on my view to talk about 'correspondence', as long as it is understood to be a metaphorical talk. It's perfectly fine to say that there are things outside our minds that we can get either right or wrong, but only we should remember that it doesn't mean that we have here two different conception of reality (thinking about things 'from inside' our minds, and thinking about them as they really-are-in-themselves), but only one which is cashed out in different words (but it doesn't mean that we have to choose either of them, and declare the other as false or nonsensical, but rather we must seek a middle way here).

In other words, Truth-by-correspondence about everything as a whole is neither right or wrong, but meaningless because it is unthinkable, so that neither skepticism nor non-skepticism in this sense is strictly meaningful. Isn't that the case? — sime

I'm not sure that I understand what you mean.

As i previously suggested, i suspect that some dream skeptics, possibly most of them, are implicitly defining the "dream vs reality" distinction in terms of the coherence and cohesiveness of their experiences - which is of course an entirely internal notion to experience that is both understandable and doesn't involve any Cartesian notion of transcendental truth-bearers beyond the individual's experience. — sime

To tell you the truth, it doesn't strike me as very plausible to say that skeptics are in fact closet coherentists. In fact, probably the most popular argument against coherentism (both as an epistemic theory and a theory of truth) precisely exploits the idea that coherence is not a guarantee of truth. You can imagine a subject with a maximally coherent web of beliefs, which nevertheless are all systematically false, which means the two concepts can't amount to the same thing.

Of course it is true that we often judge what is true and false on the basis of internal coherence with the rest of our beliefs, but attempting to define the distinction between appearance and reality simply in terms of coherence seems to me wrong.

It seems to me that what you are proposing is some form of anti-realism, but that would be to surrender to the skeptic, rather than answer him. Because it looks like that you essentially agree with his main contention - that what we can know, or what is cognitively accessible to us (or even meaningful) is bounded by the limits of our consciousness, and there's no such things as going beyond it (or going beyond all evidence). And if we accept that assumption, it would be of little comfort to go on and define truth in terms of 'internal coherence' of mental states, or something of that sort. Obviously there's something right about the common sense idea that what we believe to be the case and what is really the case are two different things (at least conceptually, though they can sometimes coincide), and that there are no foolproof methods to ensure for 100% that we cannot go wrong in what we believe. And so in my opinion giving up this natural idea is just a too high price to pay.

On the other hand, I do agree that there's something right in anti-realism or verificationism, and it is in the basic idea that cognition cannot go 'outside of itself', and therefore it is right to say that cognition does in some sense has a limit, but - and this is the crucial point - it is limited from the 'inside' and not by something from the 'outside' (and I'm alluding here to Wittgenstein's discussion of limits in Tractatus, e.g. the preface). The wrong idea would be to think about the limits as psychological limits (like our mental states), that is, contingent limits that only happened to be placed on us by nature (and are discoverable by philosophical reflection). But what I mean by an 'internal limit' is simply to say that (trivially) what is thinkable is limited by what is thinkable, and by "thinkable" I don't mean merely a psychological phenomenon, but the limits which are set by our concepts or logic, which define what makes sense to us. This however is not to give up the common sense idea that I've described earlier, of the distinction between how reality is and what we believe it to be, but it does mean that we have to radically rethink what it amounts to (and by that I mean that the picture which accompanies thinking about 'realism' in philosophy is confused, and doesn't give us what we think that it gives).

So on this alternative conception, what is confused in skepticism is the idea that 'reality in itself' and our knowing reality in itself' are conceptually completely different things. But on the alternative conception that I'm proposing, how things are 'in themselves' is not different from that which we know, or can know, or at least imagine ourselves of knowing. That is, there's no other perspective on reality available to us, different from the perspective which is made available to us in successful instances of knowing (or at least what we take to be successful instances of knowing). So the confusion in the skeptical argument lies in the fact that the skeptic thinks that asking what is really the case independently of us, is a completely different question from asking what we take ourselves to know to be the case. And this is what I tried to bring out in my distinction between 'waking' and 'dreaming' states. The contrast between the two is logically dependent on what we take to be available to our experience in successful waking states. But if successful waking states were not at least conceptually conceivable, then neither dreaming or illusory states would, since the letter are simply defined in contrast to the former (and this just follows from how the skeptical argument itself is set up structurally).

So I'm not trying to obliterate here the distinction between appearance and reality (or knowledge, and the facts that we know), since it doesn't follow on my account that if we think that we know that p, then it must be the case that we really know that p. But what I am saying is that what reality is (in the strong metaphysical sense of 'things-being-in-themselve-independently-of-our-minds') is precisely that thing which we imagine ourselves to know if indeed we know it and are not mistaken (and I want to strongly emphasize here the 'if' clause, which is what sets my position apart from anti-realism).

(I apologize for the rambly comment)

Does one's working definitions of Waking and Dreaming reduce to immediate empirical contents, to non-immediate empirical implications, to both or to neither? — sime

I'm not quite understand the question, but the basic idea is that a waking experience is a state where you experience reality as it really is (so if you are awake and see a tree, then it follows that there's really a tree in front of you and so on). Dreaming experience on the other hand is a sort of counterfeit waking experience - everything looks as if you are having a waking experience, when you really aren't.

Do the sets of experiences referred to by one's working definition of waking and dreaming overlap, or do waking and dreaming refer to disjoint sets of experiences? — sime

Yes and no. They differ in that in the one you experience reality as it is, while in the other you don't. On the other hand, they are indistinguishable on the subjective level as the skeptic maintains (but on my account, they still differ in their epistemic significance, which is how I purport to block the skeptical conclusion).

The idea that there is an objective reality independently of the mind, which you none the less, have access to is a contradiction, because reality can't be independent of you and yet you have access to it. — Mikkel

Why not? That's a really extravagant claim to make for someone who calls himself a "skeptic".

Reality is your experiences and your beliefs about how to make sense of it. There is no "The objective reality", because then subjectivity is not a part of reality. — Mikkel

This simply begs the question. If this is meant as an argument to prove what you said in the first quote ("mind independent reality is a contradiction"), then you cannot start from a premise which equates reality with your experiences. You indeed experience reality through you subjective states, but it doesn't follow that reality is identical with them.

That facts, evidence, truth, knowledge and so on matter to you is because, that it matters to you, is what makes it subjective. — Mikkel

What you say here doesn't follow. If dogs matter to me subjectively, that doesn't prove that dogs themselves are 'subjective'.

You completely misunderstood my post, that's all that I can say.

Now I see a tree. That is non-inferential, I will accept that for now. But what that have to do with the objective reality? What do you mean by objective reality and how do you know that there is such a thing as an objective reality? — Mikkel

"Objective reality" simply means that things are exactly as they appear to you perceptually (when you see a tree, there's really a tree in front of you etc.). Really, there's nothing to explain here.

And about the "how do you know?" part, it's a bit complicated, but I'll try to keep it simple. The basic idea behind the skeptical argument (that is, the argument from illusion) is that even what we take to be the best cases of knowledge aren't really knowledge because we cannot rule out every conceivable possibility of us being wrong. So we have two options according to the skeptic for ways for the world to turn out to be: for every imaginable experience (say of seeing a tree) either a. things in the world match exactly the way they appear to you or b. you are completely deceived about everything in your experience since you are perpetually hallucinating or a brain in a vat etc. And since no piece of perceptual evidence could support favoring possibility (a) over (b), you cannot know that you are living in (a) and not (b) (and hence you can't know anything).

But notice the difference between scenarios (a) and (b). Suppose that scenario (a) is the actual one, what this world would be like? Well, that would be a world in which things are almost exactly correlated with the way they appear to you perceptually, at least in the typical cases (since the skeptic allows us to choose any case we wish as the best candidate for knowledge; and as I've argued before, for every experience of things appearing to you to be such and such, there's a possibility that things in the wold are indeed such and such, even if they are not so in the actuality). And now let's consider world (b). The way the example set up, world (b) is designed in such a way that things look to you exactly as if you are in world (a) (but you aren't), and you have no way to tell (based on your experience). So in this world, reality has absolutely nothing to do with the way things appear to you, but somehow, by virtue of some miracle or extremely clever design, your appearances are correlated with the way the world would've been if world (a) were the actual one (because otherwise it would not be a case of an illusion).

And now, keeping all this in mind, let us assess the relative probabilities of worlds (a) and (b). Given that you are having right now an experience as if you are living in an (a) type world, what are the odds that you really are? I think they are pretty high (or even extremely high - and conversely, they are very low for (b) being the actual world), and this is shown by the mere fact that in order to persuade you that you might be mistaken, the skeptic had to concoct this very bizarre and unlikely sci-fi story about brains in a vat or whatever. But the world as you perceive it, is such (if we assume for a second that it is the actual world) that in it scenarios of the kind that the skeptic envisions are extremely unlikely; that is, cases in which reality matches appearances are way more common than when it doesn't (because by hypothesis, it is a world where whenever you see a tree there's really a tree, since normally no one tries to deceive you etc.). And that means that it is much more likely for your experiences (taken by themselves) to turn out to be right than wrong, since it would not be a coincidence if your appearances corresponded with reality, but it would be an extreme coincidence if (despite their internal coherence etc.) they actually didn't.

So I believe that we should conclude that we have a very good reasons to favor the belief that we actually live in world (a) rather than (b), since given the extreme unlikeliness that all our experiences are systematically deceptive (as the skeptic imagines), we are perfectly entitled to ignore such possibilities and trust our senses at face value. And now if you supplement all this with my previous account of non-inferential justification, you can actually get from all of this an account of a genuine perceptual knowledge of the world.

I didn't mean to say that hallucinations are 'unreal' in your sense, relax...

When you have a hallucination of course your experience is as real as any (and that it is a state of your brain etc). The point is rather that not all experiences are necessarily hallucinations, since the very concept of hallucination presupposes that it's at least also possible to perceive directly how things are in the objective reality (and by "unreal hallucination" I just meant a genuine experience - e.g. seeing a real tree as opposed to just hallucinating a tree etc. - I didn't mean that the experience of hallucinating itself is not real).

Now if you claim that hallucinations are not real, then I have to ask - How do you know that? — Mikkel

They are sometimes real, but not always (at least they need not be, or you can't prove that they have to). I tried to show in that other post on skepticism that hallucinations (for their intelligibility as hallucinations) presuppose that there's also a possibility to perceive things just as they are in reality. Think about it this way: what makes something into a hallucination? The fact that when you have it, it looks to you as if things around you are in a certain way (when in fact they aren't). But this means that there's also the possibility for things to be in the world just the way they appear, which will not be a state of hallucination, since your experience will portray accurately the way things are in reality (so if it is possible for your experience and the world no to match, then it is also possible - at least logically - for them to match).

This is the first step. The second step will be to show that we are justified to rule out or ignore the sorts of hallucinations or illusions of the kind to which the skeptic appeals in his argument. I also discussed this to some extent in the other post, though a bit more needs to be said about that, since people didn't get the idea (I can elaborate on this if you wish).

You have made a naive realistic claim - "You know that you perceive something by perceiving it, how else?", but you have given no evidence. — Mikkel

The evidence is simply my perceptual state (of seeming to see that something is the case in the world).

Again, you are misunderstanding what 'non-inferential' means. It means that your evidence is simply the state on which you base your belief. And you don't need further evidence (to prove that your original evidence is good, or is not the result of an illusion etc.) precisely because your experiential state has intrinsic justificatory power to support what is believed, which is independent of any further premises (hence it is called 'non-inferential').

How do you know that is something and not an illusion? — Mikkel

Now you are just using the Cartesian argument which I discussed in my other post. I thought that your 'trilemma argument' was a different argument from the classical argument from illusion (and by the way, you said previously you agreed with me that the argument is incoherent).

That you are perceiving them!
How do you know that? — Mikkel

What do you mean? You know that you perceive something by perceiving it, how else?

How do you know that? — Mikkel

Know what?

I don't know ;) how to get this across. It is pointless to point out that my argument about knowledge fails, if all arguments about knowledge fail. — Mikkel

I don't understand what you mean. If your argument fails, then it fails, which means that you haven't proved your conclusion (skepticism or whatever it was). So it's not pointless.

That you believe you have knowledge, means you have knowledge? Is that your point? — Mikkel

No, my point is that your reasons for denying that we have knowledge aren't very good (and I'm not trying to prove to you that I do have knowledge - I've got better things to do).

Further you claimed that there are things, which are know non-inferentially, so would you please explain, how you know that? — Mikkel

I already told you - by perceiving them. This is what 'non-inferential' means - the reason for your belief is not in the form of an argument which you can give to someone, it just suffices to have the right sort of experience without needing any additional reasons.

What you said was this: "I know, that you can know many things non-inferentially."
For which I answer: "How do you know that?" — Mikkel

My point is that your argument doesn't prove that we don't know things non-inferentially, and since most people believe that they do know things on the basis of their sense experience (and not arguments), your argument simply doesn't engage the most plausible view out there regarding knowledge. I'm not trying to assert that I'm right that we do know things non-inferentially, I'm just saying that your argument doesn't show that we don't (and so I don't have to prove that we do, in order to show that your argument doesn't succeed).

But that's not what I'm saying. I'm not saying that your argument is wrong because I know that there is such and such in front of me because I can see. My point is that you argument makes an unwarranted assumption, that all reasons to believe must take the form of an argument. Of course arguments have to stop somewhere (as the regress problem shows), and my claim is that sense experience would be a very plausible stopper. It does not follow however that stopping at sense experience deprives you of the right to claim that you know (or that it would be dogmatic to do so), since again, I don't see any reason to assume in advance that arguments are the only legitimate grounds for knowledge.

When you claim something, you ask yourself how you know that. Then you make a reasoned argument about that — Mikkel

The obvious answer to this is to say that not all knowledge is based on arguments with premises as you claim. You can know many things non-inferentially, say by basing your beliefs on a perceptual experience which you take to reveal to you directly how things are in the world, and not by virtue of being part of some argument.

because all such claims run into Agrippa's Trilemma. — Mikkel

What's "Agrippa's Trilemma"? Would you mind explaining?

You completely misunderstood my argument (from that other thread on skepticism). I didn't assume that we have knowledge, or that there is a world, but I made an internal criticism of the skeptical argument, and that's a different thing. But I don't want to go into details since I already explained the main idea in my OP, and I don't think it's appropriate to discuss it here.

It is funny though that you can't spot the problem in your own claim to knowledge. — Mikkel

Where did I make such claims?

They say nothing about doubt or knowledge. They only say that it's more likely that we're simulations or Boltzmann brains. — Michael

But what does a 'simulation' mean? Doesn't it entail that all your experiences are illusory?

As philosophical arguments for skepticism, the two arguments are plainly incoherent. It doesn't make sense to doubt our knowledge of the external world (or its existence) by appealing to aposteriori premises that themselves could be known only if we presuppose that we do have knowledge of the external world (or that it exists).

The answer is that knowing about the world is not a matter of knowing what causes your beliefs/experiences. It is true that (as a matter of scientific fact) we have experiences because of the causal impingement of the world on our senses, but it doesn't follow that we have to know anything about those causal processes to have knowledge about the world.

No, evidence is apprehended as being correlated with the belief which it is evidence for. So you have two things wrong here. First, the thing which the evidence is evidence of, is a belief it is not a fact. It cannot be called a fact, because the purpose of evidence is to convince someone of something which may or may not be true. Second, in order for it to be called evidence, it need not be intimately related to the belief, it needs only to be perceived as such. This is what makes it evidence of the thing, the fact that it is perceived as being related to the thing, whether or not it actually is, is irrelevant. — Metaphysician Undercover

Evidence is connected to belief via its relation to the facts. One usually believes that such and such is the case because the evidence tells him that such and such is the case. If the evidence didn't indicate anything about how things are in the world, it would make no sense to believe things on their basis. If you didn't believe that e.g., having an experience of seeing a tree is somehow connected with the presence of trees, then you would not take your experience as grounds to believe that there's a tree.

This doesn't make any sense to me. You seem to be using "justify" in a strange way. We often claim to know something when someone we trust has told us that. But this is not at all a form of justification. So we often claim to know something, and have a reason for making such a claim, yet that reason doesn't constitute justification. — Metaphysician Undercover

I don't understand what you are saying. If you are not justified to believe p on the basis of E, then E is not a justification, or a bad justification, or a bad reason (however you want to call it) to believe that p. You can't have it both ways. It's nonsense to say that you have good reasons to believe that p but you are not justified to do so, or vice versa (and I'm treating here 'reasons' 'grounds' and 'justification' interchangeably).

I've already told you, as well as javra has told you, that this is a misrepresentation of skepticism. In my last post, I clearly pointed out, in your own argument, how what you say here is not true to your argument. — Metaphysician Undercover

But you have said this very thing yourself in one of your earlier comments, quote: " because you acknowledge that your judgement is fallible, then you have no way of knowing whether your judgements are knowledge or not. Therefore you should doubt all your knowledge", which comes to the same as saying that one doesn't have grounds or justifications to claim that one knows something, which is precisely what I said.

The argument, as you presented it, is that if knowing something requires absolute certainty, then we do not know anything. It does not say that you cannot know anything unless you are absolutely certain. — Metaphysician Undercover

How come? It follows logically...

If S knows p only when S is certain that p - then, the conjunction that S knows that p and S is not certain that p is always false (that is, necessarily false).

If P -> Q

then it is not the case that P & ~Q.

Simple logic.

That's a ridiculous definition of "evidence". Evidence supports a belief it does not render it impossible that the belief is false. That's why to convince someone of something it usually requires more than one piece of evidence. If evidence for a belief rendered the belief necessarily true, then all that would be required would be one piece of flimsy evidence and the belief would necessarily be true. — Metaphysician Undercover

Fine, if you don't like the definition, then you can weaken it, e.g., evidence is something that makes what is believed more probable or likely to be true than not - it doesn't change the main idea. What is crucial is that for something to be 'evidence' it must be intimately correlated with the facts that it is evidence for.

I think you misunderstand skepticism. The skeptic doesn't claim that our confidence ought to be zero, the skeptic claims that the confidence cannot be one hundred percent. And, since we cannot have absolute, one hundred percent confidence in any claim of knowledge, all knowledge ought to be doubted. — Metaphysician Undercover

If your grounds for claiming that you know something don't justify you to say that you know, then I think it comes down to the same thing as saying that you don't have any good reasons at all to say that you know (and hence you ought not to have any confidence in your knowledge whatsoever).

Again, I think you misrepresent skepticism. The skeptic does not think that the belief is entirely without grounds, the skeptic thinks that the grounds for the belief ought to be examined. — Metaphysician Undercover

If the belief is not entirely without grounds, then why are they not good enough to be considered knowledge? The structure of the skeptical argument is such that all of your claims to knowledge are completely worthless unless you've ruled out all possibilities of error. If he doesn't assume that, then it is simply not clear how all those sci-fi stories about deceiving demons and brains in a vat are supposed to prove skepticism (that we don't know that there's an external world etc.). Obviously the skeptics' argument is based on the idea that you can't say that you know something unless you are not absolutely sure that you are not mistaken - and of course this is the assumption that I'm rejecting.

But very often people mistake shrubs for trees, and trees for shrubs. It is a common mistake. A small tree might be mistook for a shrub, and a shrub might be mistook for a small tree. How do you rule out this possibility for error unless you know that the person is adept in this type of judgement? — Metaphysician Undercover

You rule it out by learning to identify shrubs, if they are very common where you live. Or alternatively, if you are living in an environment where there are no shrubs that look just like trees, then you can know that something is a tree without knowing anything about shrubs, or to be able to identify them. What are the relevant alternatives that one has to rule out is very much depend on the context.

Skepticism is not a claim that knowledge is "completely groundless". It is the claim that the grounds are just as likely to be mistaken as anything else is. — Metaphysician Undercover

Which is the same as saying that you have no grounds. If your grounds to believe that a certain event will happen are as good as your grounds to believe that the event will not happen, then you simply have no grounds whatsoever to believe either outcome (since for all you know, the probability is 50/50).

What would be the point in lowering the standards for knowledge? You seem to think that this would get rid of the skeptic, but actually the reverse is true. If the standards are lowered, we can say P is knowledge when we have a lower degree of certainty of P. This means more cases of what is called knowledge turning out to be false, giving us more reason to be skeptical of anything which is called knowledge. — Metaphysician Undercover

I think that no person in the world actually believes that he's infallible like God, so 'lowering the standards for knowledge' simply means adopting the standards that all reasonable people adopt anyway, so this ought no to be a problem.

The argument concludes "no p can be known". It defines "know" as ruling out the possibility of error, in premise 1. The argument says nothing about degrees of confidence in one's belief. The skeptic doesn't say, as you claim, that we can have zero confidence, the skeptic says that if "knowing" requires ruling out the possibility of error, as per premise 1, then we cannot know anything. This does not say that we cannot have any confidence in our beliefs. It says something about the nature of "knowing". — Metaphysician Undercover

I should qualify that by 'confidence' I meant 'justified confidence' in the epistemic sense. Of course the skeptic doesn't dispute that most people, as a matter of psychological fact, are confident that they know many things; his claim is rather that they ought not to be confident in that, because they don't really know anything, properly speaking. And my response was to claim that we actually do have much better reasons to be confident in our beliefs than what the skeptic thinks (and recall that it was a response to what you argued in your previous post, that since all our judgments are fallible, we can't know that what we have is knowledge, and I tried to show that it doesn't follow).

To found my statements in fact, both Plato and Hume held that there is an external world. Both were staunch philosophical skeptics, rather than parodies of what philosophical skepticism entails. — javra

Maybe you are right about Plato and Hume, but I'm not concerned with the views of any particular philosopher, but with a generic view (or rather a form of argument) which is called 'skepticism' in contemporary analytic philosophy (of course it doesn't mean that you cannot call other things 'skepticism', but that would be a different use of the world from the one that interests me in this thread).

The underlined portion of the quote is exactly what the (philosophical) skeptic is saying: that there is no absolute certainty, knowledge, or truth that we can apprehend, only optimal approximations of absolute certainty, absolute knowledge, or absolute truth - which is not the same as not having standards at all. — javra

No, he doesn't say that - rather he's saying that there's no such thing as knowledge of the external world as such. See the conclusion of the skeptical argument in my first post. If what the skeptic was trying to prove is that absolutely certain knowledge is impossible, then that would not be a very interesting claim - but what makes his argument interesting is that he's claiming that there's no such thing as knowledge, no matter what standards of knowledge you adopt (that is, within the bounds of what we would be inclined to call 'knowledge').

My point, is that the evidence, your perception of a tree, never provides the basis for a conclusion which beyond the possibility of doubt. If you exclude all the cases in which you were wrong, i.e. it turned out to be a shrub or something like that and not a tree, to support your claim that the judgement is beyond doubt, then you are being unrealistic. — Metaphysician Undercover

It depends on what one means here by "evidence". On my understanding, having evidence for p is being in a state of such kind that you cannot be in this very same state when p is false. So when you perceive a tree in full daylight, your vision functions properly etc., then your visual experience of the tree is correlated with the fact that there is a tree (you cannot after all see a tree if there's no tree - of course you can mistakenly believe that you are seeing a tree, but the point is that if that happens then you're in a different kind of state).

Since "knowledge" as you use it refers to a successful judgement, and you have no way of knowing whether your judgement is successful or not, because you acknowledge that your judgement is fallible, then you have no way of knowing whether your judgements are knowledge or not. Therefore you should doubt all your knowledge. — Metaphysician Undercover

I think we should separate here the question about whether you know that p, and your degree of confidence in that knowledge (which I think are two different things). Indeed the thought that all our judgments are to some degree fallible may undermine our confidence in what we think we know (particularly when we think about the skeptical problem), but it doesn't have to. The skeptic says that our confidence in all of our claims to knowledge ought to be 0, since there are some crazy possibilities of error that we cannot rule out. But does it follow? I think not.

Think about it this way. Suppose that you are having an experience of seeing a tree in broad daylight etc., and also you don't have any particular reason to doubt that something is wrong with you - i.e., you don't remember taking drugs, or being told by the doctor that something is wrong with your eyesight and so on. Now, suppose that you form a belief that there's a tree in front of you on the basis of your experience. Is your belief entirely without grounds as the skeptic claims? I don't think so, because the experience that you are having does rule out objectively, some possibilities of error - e.g., that you are looking at a traffic light or a painting of a tree etc.- since it is impossible for a person with a normal eyesight and proper lighting conditions to take a painting or a traffic light for a real tree. So it is simply not true that your relying on the experience that you have (and given the background of your other beliefs) to be completely epistemically unjustified as if you've made a random guess (as the skeptic wants you to believe). Now, the skeptic will insist that there are some possibilities that your experience doesn't rule out - like for example you being a brain in a vat your whole life in a treeless world (which is true as I admitted). But the question is, why should we worry about such possibilities?. Suppose that (for some reason) you are confident that such a possibility is extremely unlikely and far fetched, and you just ignore it for all intents and purposes. Does it make you completely epistemically irresponsible or unreasonable? I think that no, since you are still very careful not to believe things which are plainly inconsistent with your actual experience - i.e. you don't form the belief that you are seeing a street light when you have an appearance of a tree etc., so you are objectivly excluding some possibilities of error, and you have excellent reasons to believe that.

So in pother words, what I'm trying to argue is that we have good grounds to trust our judgments (and take them to be knowledge) even in the face of their fallibility, since it is simply not the case that they are completely groundless. Why should we assess our judgments relative to the imaginary stories that the skeptic tells us? If we just stop being obsessive about absolute certainty, and adopt some more modest standards for knowledge claims (which is not the same as not having standards at all), then there will remain no longer any good reason to worry about what the skeptic is saying, and thus no reason to not to be confident in most of our claims to know.

I don't see how you can make this conclusion. A person in a waking state may have poor eyes, poor judgement, or be hallucinating when thinking that they are in a state of perceiving a tree. — Metaphysician Undercover

I meant to exclude such cases, of course there are many ceteris paribus conditions that we must take into account. I meant that when you perceive a tree (you are not dreaming, your eyesight is normal etc.) then your perceptual state is correlated with the fact that there's a tree in front of you, and this is an objective matter. This is what should be properly regarded as your evidence.

You seem to be neglecting the fact that evidence must be judged. The person must judge the perceptual evidence, as well as the meaning of the statement "that is a tree", in order to know that that is a tree. Human judgements can be mistaken. Therefore the person can be wrong. — Metaphysician Undercover

My judgment is fallible, but it doesn't mean that my perceptual evidence is fallible, which is what I'm insisting on.

So here you use "objectively entail" to refer to the judgement which must be made. How do you ensure that the human judgement is not mistaken? — Metaphysician Undercover

No, it doesn't refer to my judgment. You know that p, if you judge that p on a basis of evidence which entails that p (and p is true). Such judgments are fallible as you say, but it doesn't show that they are not knowledge when they do succeeded.