Tuesday 11 June 2019

David Lewis's infallible Psilosophy

David Lewis's paper 'Elusive knowledge' assumes that knowledge can be, not a Tarskian primitive term- i.e. something undefined within a specific universe of discourse- but a term defined in terms of infallibility across all possible worlds. The problem here is that since 'infallibility' has become the undefined term- whose necessary conditions are unknown- how can we know when its conditions are met? In any case, why have a discourse whose primitive term is infallibility? What use would it be? In ordinary speech we ask questions like 'do you know how to drive a car?' or 'do you know Sanskrit?' We don't ask questions like 'is this car infallible?' or 'are Sanskrit sentence infallibly comprehensible?' Why is this? The answer is that a guy who says 'this car is infallible. Even if all its wheels fall off, you can still drive it' is clearly either lying or does not know what he is talking about.  I mention Sanskrit because I once wrote a blogpost about a guy who claimed that all sentences in Paninian Sanskrit are infallibly comprehensible!

It is easy enough to check if a guy really knows how to drive a car or speak Sanskrit. That is why 'knowledge' is a good primitive term for a universe of discourse. Infallibility, on the other hand, is a terrible primitive term because nothing about it can be known- at least not infallibly.

Lewis wrote-
no sooner do we engage in epistemology - the systematic philosophical examination of knowledge - than we meet a compelling argument that we know next to nothing. The sceptical argument is nothing new or fancy. It is just this: it seems as if knowledge must be by definition infallible.
If it is a primitive term then, by definition, it has no definition. It may be anything at all. One can't say.
If you claim that S knows that P, and yet you grant that S cannot eliminate a certain possibility in which not-P, it certainly seems as if you have granted that S does not after all know that P.
A thing may seem one thing and be another.
To speak of fallible knowledge, of knowledge despite uneliminated possibilities of error, just sounds contradictory. 
Sounding contradictory does not make it actually contradictory.

Substitute, in the above, 'physics' for 'epistemology' and 'scientific' for 'philosophical' and 'P' for 'the force of Gravity' and look at the result.
no sooner do we engage in physics - the systematic scientific examination of forces - than we meet a compelling argument that we know next to nothing. The sceptical argument is nothing new or fancy. It is just this: it seems as if knowledge must be by definition infallible. If you claim that S knows something about the force of Gravity, and yet you grant that S cannot eliminate a certain possibility such that the force of Gravity does not exist, it certainly seems as if you have granted that S does not after all know anything about the force of Gravity. To speak of fallible knowledge, of knowledge despite uneliminated possibilities of error, just sounds contradictory.

I think guys at NASA knew a lot about gravity. They may also have known that there may have been a time when the force of Gravity did not exist. Indeed, it may be that there is some probability that the force of Gravity might cease to exist either locally- permitting the use of anti-grav devices like the 'hover-board' in Back to the Future- or at the cosmological level. But this does not mean that nothing is known about Gravity.

It is a different matter to say that Physics has no a priori truths. Armchair physics does not become knowledge till experiment or experience confirms that the new theory has superior predictive power and makes possible the invention of very useful new devices.

What is true of Physics must also be true of metaphysics because unless the latter can come up with new knowledge in the former, it has not gone beyond physics at all. It is merely nonsense. It is no good saying- 'Metaphysics tells us which Physics theory are licit'- because the historical record shows no such thing. At one time, it might have seemed otherwise- Einstein, at a certain stage, certainly spoke like a philosopher - but no philosopher has actually contributed anything at all to physics for 200 years. Mathematicians- sure. Artists of various types- arguably so. Philosophers? Don't be silly.

As Chad Orzel puts it-
When physicists like Lawrence Krauss and Stephen Hawking speak dismissively of philosophy, they're usually talking about stuff in the "this offends my vague intuitive sense of how the universe ought to operate" mode of Einstein's later dissatisfaction with quantum mechanics (and to the outsider, vague intuitive dissatisfaction can seem like a major component of a lot of academic philosophy). 
It's worth noting that both of these approaches have some use. The rigorous and precise thinking that led to relativity is obviously very useful and important to physics. The fuzzier sort of philosophizing that Einstein and Bohr were prone to is also useful, albeit indirectly (in joking physics jargon, it's second-order useful)-- their poking at ideas around the philosophical foundations of quantum physics didn't produce much that was directly useful, but did provide some inspiration for later thinkers like Bell. Bell was able to provide the rigor and precision that Bohr and Einstein's arguments lacked, and express the key concepts in clear mathematics that help drive the modern revolution in quantum technology.
Before returning to David Lewis's paper, I would like to introduce a highly useful distinction- that between a cheese knife that cuts cheese and a cheese knife which is made of cheese. If you want cheese but have none you may prefer the gift of the latter type of cheese knife. On the other hand, if your task is to sample and score hundreds of different consignments of cheese, you may find a particular, non-edible, type of cheese knife a godsend.

Not let us consider a situation where there is no knowledge but lots of skepticism. You may want something which is known to cut into skepticism in an infallible manner. The reverse would be the case where there is plenty of knowledge and no skepticism. In that case you would prefer a tool which cuts into knowledge so that room for skepticism is cleared.

Philosophy, as defined by Socrates, is not knowledge but the love of knowledge. It may inflate a great many sex-dolls for its own pleasure. But such pleasure is not love. It may be that a sharper type of prick is needful so as to deflate these too numerous sex dolls and thus clear a path to something worthier of one's affection.

Davis began his essay by saying we have abundant-too-abundant knowledge which it would be absurd to express philosophical skepticism about. However, it pays smart people to be skeptical, to some practical end, of this 'everyday' knowledge. It is the secret to technological innovation and entrepreneurial or artistic success.

Davis then says 'it is a Moorean fact that we know a lot'- i.e. ' we know we know a lot better than we know the premises of any philosophical argument to the contrary.' But this is only because we know 'the premises of any philosophical argument' are as stupid as shit. Moorean facts only exist by virtue of our knowledge that what Moore was engaged in was psilosophy- 'slender wisdom'- or stupid shit.

Davis ends the first page of his essay thus-

 We never have infallible knowledge. 
Never - well, hardly ever. Some say we have infallible knowledge of a few simple, axiomatic necessary truths; and of our own present experience. They say that I simply cannot be wrong that a part of a part of something is itself a part of that thing; or that it seems to me now (as I sit here at the keyboard) exactly as if I am hearing clicking noises on top of a steady whirring. Some say so. Others deny it. No matter; let it be granted, at least for the sake of the argument. It is not nearly enough. If we have only that infallible knowledge, yet knowledge is by definition infallible, then we have very little knowledge indeed - not the abundant everyday knowledge we thought we had. That is still absurd.

So we have a reductio ad absurdum here. We granted something for the sake of argument and reached an absurd conclusion. So we ought not to grant that thing even for the sake of argument because to do so is silly.

Knowledge is fallible. We may have some provisional axioms and primitive terms to get going- at least pedagogically speaking- but those axioms are not irrefragable, nor are those primitive terms well defined at some higher 'meta' level of discourse.

Lewis waxes histrionic at this outcome-
So we know a lot; knowledge must be infallible; yet we have fallible knowledge or none (or next to none). We are caught between the rock of fallibilism and the whirlpool of scepticism. Both are mad! Yet fallibilism is the less intrusive madness. It demands less frequent corrections of what we want to say. So, if forced to choose, I choose fallibilism. (And so say all of us.) We can get used to it, and some of us have done. No joy there - we know that people can get used to the most crazy philosophical sayings imaginable. If you are a contented fallibilist, I implore you to be honest, be naive, hear it afresh. 'He knows, yet he has not eliminated all possibilities of error.' Even if you've numbed your ears, doesn't this overt, explicit fallibilism still sound wrong?
No it doesn't. We believe we evolved under Knightian Uncertainty. We can't specify all possible states of the world. Hence we can't 'eliminate all possibilities of error'. Instead, we can pursue regret minimizing strategies which feature error correcting mechanisms.
Better fallibilism than scepticism; but it would be better still to dodge the choice.
By not talking about it.
I think we can. We will be alarmingly close to the rock, and also alarmingly close to the whirlpool, but if we steer with care, we can -just barely - escape them both.
It is this claim which I will show to be foolish.
Maybe epistemology is the culprit. Maybe this extraordinary pastime robs us of our knowledge.
Or maybe the thing is silly.
Maybe we do know a lot in daily life; but maybe when we look hard at our knowledge, it goes away. But only when we look at it harder than the sane ever do in daily life; only when we let our paranoid fantasies rip.
So being mentally ill is a good qualification for this line of work.
That is when we are forced to admit that there always are uneliminated possibilities of error, so that we have fallible knowledge or none.
Who is forcing us to admit anything? Why not plead the Fifth or plead with your inquisitor for a fifth of whiskey in return for mouthing whatever nonsense it is he wants to hear?
Much that we say is context-dependent, in simple ways or subtle ways.
That which isn't being sententious nonsense simply.
Simple: 'it's evening' is truly said when, and only when, it is said in the evening.
Nonsense. There is a poem by Faiz titled 'The Dawn of Independence Day' which depicts it as the final guttering gloaming or gotterdammerung of all that had been most glorious about the Freedom struggle. Faiz, of course, was speaking from Lahore. For India, whose Freedom came at Midnight, it might have been more truly said that the dusk of the 14th of August was not 'Sandhya' but 'Suprabhata'.
Maybe ascriptions of knowledge are subtly context-dependent, and maybe epistemology is a context that makes them go false.
And maybe 'ascriptions of knowledge' are worth as much as ascriptions of virginity to a prostitute made by her elderly son and pimp. Maybe 'epistemology' is about as worthwhile as 'Development Studies' or 'Post Colonial Theory' or any other such Credentialist Ponzi scheme preying upon cretins.
Then epistemology would be an investigation that destroys its own subject matter. If so, the sceptical argument might be flawless, when we engage in epistemology - and only then! 
Skeptical arguments are flawed unless they make room for something better. Utility, instrumental value, is the lustre the lapidary adds to the diamond such that what was a flaw is now a facet.
If you start from the ancient idea that justification is the mark that distinguishes knowledge from mere opinion (even true opinion), then you well might conclude that ascriptions of knowledge are context-dependent because standards for adequate justification are context-dependent.
This is exactly what happens in first order, utile, discourses. There is a quality of economia- good faith, give and take- which is the opposite of Akrebia- which foolishly stipulates for a higher standard of precision than the subject matter is capable of yielding.

Thus, if we are judging cheeses, we may be able to reach a good faith consensus that, for example, Camembert is superior to Kraft. However, if some 'molecular gastronomist' enters the fray and begins arguing, on the basis of abstruse speculations regarding the impact of cosmic radiation upon cheeses stored on board an interstellar vessel, we would have to entertain the possibility that Kraft might superior to Camembert under those circumstances. But, by then, genuine cheese lovers would be cheesed off. They'd stop caring about judging cheeses and stick to just gorging on the ones they like in a solipsistic rapture that is mute if not morose.

Something like that has already happened to Literature. When was the last time you bought a book because it won the Booker or the Nobel? If anything, the accolade is a turn off.
As follows: opinion, even if true, deserves the name of knowledge only if it is adequately supported by reasons;
My opinion that the neighbor's cat is soooo cute, even if true, does not deserve the name of knowledge though I have reason enough for my claim- and video evidence to back it up- because few people are interested in me or my slender means of attaining a rejoicing heart.
to deserve that name in the especially demanding context of epistemology, the arguments from supporting reasons must be especially watertight;
I suppose another way of saying this is 'Epistemological arguments must have a mathematical representation with 'univalent foundations'. In other words, they must be formalizable in a manner that permits relatively quick, algorithmic, error checking. Godel's proof of God is an example of a metaphysical argument which has been refuted by a computer.

I see no great difficulty in accepting that Epistemology, even when inquiring into its own proper foundations, might have a novel logical, and therefore mathematical, representation which is highly utile.
but the special standards of justification that this special context demands never can be met (well, hardly ever).
How do we know? It does appear that Logic has developed a great deal and the thing is certainly useful. Voevodksky 'univalent foundations' approach too seems useful and productive for Mathematics. Judea Pearl's work does help ordinary people toiling in applied fields. Why can't this sort of progress continue without occasioning histrionics or scandal? A certain amount of Epistemology 'pays for itself' in diverse fields just as Maths 'pays for itself' in those same fields. Why shouldn't Epistemology be like the Foundations of Mathematics- which currently appears to 'pay for itself' in spades?

Surely, there has been some progress in the subject? At least some types of arguments are known to be fallacious regardless of context.

Lewis, still in histrionic vein, asserts otherwise
In the strict context of epistemology we know nothing, yet in laxer contexts we know a lot.
Surely, the fact that we don't consider a trial by combat to be an effective way of deciding between theories in Physics means we do know something 'in the strict context of epistemology'?

But I myself cannot subscribe to this account of the context-dependence of knowledge, because I question its starting point. I don't agree that the mark of knowledge is justification.  First, because justification is not sufficient: your true opinion that you will lose the lottery isn't knowledge, whatever the odds.
But that opinion is not fully specified. 'In my opinion, I won't win' may mean 'the lottery is rigged, I can't win' or it may mean 'In my opinion, the odds are against me' or even 'if my ticket wins, I'll still lose because I'd have died in the meantime.'
Suppose you know that it is a fair lottery with one winning ticket and many losing tickets, and you know how many losing tickets there are. The greater the number of losing tickets, the better is your justification for believing you will lose. Yet there is no number great enough to transform your fallible opinion into knowledge - after all, you just might win.
So, your opinion is a statement about likelihood and true enough in this case.
No justification is good enough - or none short of a watertight deductive argument, and all but the sceptics will agree that this is too much to demand?
'Good enough' is an economic notion. Akrebia may have no truck with it- but Akrebia misleads. It can cause any discourse- even that regarding cheese fondling- to crash.
Second, because justification is not always necessary. What (non-circular) argument supports our reliance on perception, on memory, and on testimony? 
A 'buck stopped' protocol bound juristic process- like that of the Law Courts- may solve a coordination problem in a manner which 'pays for itself'. This process may rely on 'perception', 'memory' and 'testimony'. Each may be fallible in the manner that the Law itself is defeasible. Nevertheless, the thing is useful.
And yet we do gain knowledge by these means. And sometimes, far from having supporting arguments, we don't even know how we know. We once had evidence, drew conclusions, and thereby gained knowledge; now we have forgotten our reasons, yet still we retain our knowledge. Or we know the name that goes with the face, or the sex of the chicken, by relying on subtle visual cues, without knowing what those cues may be.
What was a 'white box' may become a 'black box' because the 'instruction manual' was lost. However, if we have a good Structural Causal Model then we can, by some inspired tinkering, turn the black box white again.
The link between knowledge and justification must be broken.
Said no jurist ever when it comes to establishing guilt.
But if we break that link, then it is not - or not entirely, or not exactly - by raising the standards of justification that epistemology destroys knowledge.
It is precisely that. Akrebia is about applying a higher standard than is reasonable given the subject matter. The Courts would cease to be useful if they constantly raised the burden of proof, for example by demanding that DNA evidence be dismissed because of the possibility of there being a secret cloning facility under the Kremlin which seeks to play havoc with out justice system by framing innocent citizens.
I need some different story.
A story can 'pay for itself'- as J.K Rowling has found out. A story which can't pay for itself- like a Single Woman who tells herself she can pay the bills by using Witchcraft or Wizardry- is just a stupid lie.

Can Lewis's new story 'pay for itself'? Let us see.

To that end, I propose to take the infallibility of knowledge as my starting point.
How do you infallibly know that you do any such thing? Maybe what you are actually proposing is something very indecent to the Hypokeimenon of the Nicaraguan horcrux of the neighbor's cat. Prove otherwise Lewis or face the wrath of Chairman Meow! Oh. I see. My warning has come to late. Lewis is dead. All hail, Meow the Merciless!
Must infallibilist epistemology end in scepticism?
Not infallibly. D'uh!
Not quite. Wait and see. Anyway, here is the definition. Subject S knows proposition P iff P holds in every possibility left uneliminated by S's evidence; equivalently, iff S's evidence eliminates every possibility in which not-P.
But proposition P is not infallibly known to always be P unless it itself that proposition in which case every S's evidence eliminates it unless S is as stupid as shit.
The definition is short, the commentary upon it is longer. In the first place, there is the proposition, P.
Not infallibly. Indeed, most of the time there is no such proposition because nobody is stupid enough to formulate it. Indeed, it may be that it can't be formulated save within a stupid availability cascade which is wholly evanescent.
What I choose to call 'propositions' are individuated coarsely, by necessary equivalence.
So are what you chose to call cats or their Nicaraguan horcruxes.
For instance, there is only one necessary proposition.
Which no one has ever found necessary.
It holds in every possibility; hence in every possibility left uneliminated by S's evidence, no matter who S may be and no matter what his evidence may be.
But only because ex falso quodlibet. From what is false, everything necessarily follows.  Thus is the inexorable will of All Merciless Chairman Meow!
So the necessary proposition is known always and everywhere.
By nobody and nothing.
Yet this known proposition may go unrecognised when presented in impenetrable linguistic disguise, say as the proposition that every even number is the sum of two primes.
A proposition is what it says it is. There is no 'linguistic disguise' in the above. To say 'even numbers are divisible by 2 with no remainder' and 'even numbers are the sum of two primes' are not disguised versions of each other. An odd number may be the sum of two primes but is not divisible by 2 with no remainder.
Likewise, the known proposition that I have two hands may go unrecognised when presented as the proposition that the number of my hands is the least number n such that every even number is the sum of n primes. (Or if you doubt the necessary existence of numbers, switch to an example involving equivalence by logic alone.) These problems of disguise shall not concern us here. Our topic is modal, not hyperintensional, epistemology.  
If your epistemology has 'univalent foundations' intensionality or hyperintentionality coincides with modality. The plain fact is Lewis has no hands. He is dead. Had he encountered a shark prior to dying it would have been a known proposition that he had two..no, wait, just one hand...cancel that, Lewis has no hands and...fuck... no head either. Jeez I'm gonna barf.
 Next, there are the possibilities. We needn't enter here into the question whether these are concreta, abstract constructions, or abstract simples. Further, we needn't decide whether they must always be maximally specific possibilities, or whether they need only be specific enough for the purpose at hand. A possibility will be specific enough if it cannot be split into subcases in such a way that anything we have said about possibilities, or anything we are going to say before we are done, applies to some subcases and not to others. For instance, it should never happen that proposition P holds in some but not all sub-cases; or that some but not all sub-cases are eliminated by S's evidence.
This is like specifying that a configuration space can't co-evolve with anything within it. Why would human beings- whose configuration spaces co-evolve with them precisely because they are included in them- want to stipulate for anything so stupid?

We know we evolved by natural selection. This means that, on our own family tree, there was some 'sub-case' where a mutation falsifying 'P' occurred.

Socrates himself speaks of the 'palinode' as the method of Philosophy which saves it from the tedious labyrinth of Sophistry.

It Epicurean atoms can swerve, why can't our Epistemological phantoms?

It is a Moorean fact that Lewis type 'knowing' is fallible. We expect that some 'sub-case' will emerge which can't be eliminated by S's evidence iff S is as at least as smart as us. That's why we fund Scientific Research Programs. Otherwise, we might as well use tax-payers' money to fund the Church of the Nicaraguan horcrux of my neighbour's cat on an equal basis with NASA. Even so, a new 'P' will arise- by the Grace of All Merciless Chairman Meow, to the Glory of whose name all Praise properly belongs.
But we do need to stipulate that they are not just possibilities as to how the whole world is; they also include possibilities as to which part of the world is oneself, and as to when it now is. We need these possibilities de se et nunc because the propositions that may be known include propositions de se et nunc. 
Either genidentity exists- i.e. a being exists through time in which case a proposition known at time x which is discarded at time x+1 is considered to have failed- or else we live in a momentary 'kshanikavada' universe. In that case, as the Buddha said, only intention exists albeit ephemerally. De se et nunc knowledge is an intention of an atomic type. But it has no intensional form. Why because complex things can't exist. They are delusive simply.

For Buddhism this permits a distinction between de se et nunc omniscience (similar to the kaivalya of the Jains) with no further intention of any sort which characterizes the pratyeka Buddha, and the de se et nunc intention to postpone salvation by a pratyeka who decides to be a Boddhisattva.

Jainism gets round this type of problem by adopting something like a cautiously instrumental 'fuzzy logic' (syadvada) and considering 'essence' to be co-evolving (parninami dhravya). Hinduism- but don't get me started on Hinduism. I must hearken to Chairman Meow.
Not only do I know that there are hands in this world somewhere and somewhen.
You don't know this infallibly.
I know that I have hands, or anyway I have them now.
Not infallibly. They may have been amputated and you suffer from phantom limb syndrome. Alternatively, you may have been hypnotized.
Such propositions aren't just made true or made false by the whole world once and for all. They are true for some of us and not for others, or true at some times and not others, or both.
Why not go the whole Vedanta or Anekaanthavada hog? Why not say it is true because false for some of the all which defines us as each other while Chairman Meow purrs the Cosmos into existence?
Further, we cannot limit ourselves to 'real' possibilities that conform to the actual laws of nature, and maybe also to actual past history.
Sure we can. There may indeed be a 'total reflective equilibrium' for co-evolved systems though we know, from the theory of complexity and computability, that the thing is inaccessible algorithmically. Indeed, it is a Moorean fact that if the Universe is a 'block' then there is a unitary operator such that ergodicity obtains. This too is an equilibrium concept. Philosophy could, like Samuelson's Economics, say 'our subject has content so long as such ergodicity obtains. It is the 'total reflective equilibrium' we try to describe'.
For propositions about laws and history are contingent, and may or may not be known.
Indeed. Most would be radically 'uncertain' and feature undecidable parameters with respect to our event ontology.
Neither can we limit ourselves to 'epistemic' possibilities for S - possibilities that S does not know not to obtain.
Indeed. But Lewis's next line is wrong
That would drain our definition of content.
What drains a definition of content is its not using 'primitive terms'. The fact that S is a cretin is irrelevant. Lewis refused to accept 'knowledge' is the primitive term for Epistemology. That is why he is now leading us down the garden path.
Assume only that knowledge is closed under strict implication. (We shall consider the merits of this assumption later.)
We can't make any such assumption in good faith. Why?  We know enough math to understand that we have only a dim inkling of what 'closed' actually means. 'Implication' is even more complicated. It is likely that 'clopen' is closer to what Lewis meant- or else he was talking nonsense simply.
Remember that we are not distinguishing between equivalent propositions.
And thus setting ourselves up for a pratfall. Lewis didn't actually give us any examples of equivalent propositions. They were simply propositions he, falsely, believed to be equally true.
Then knowledge of a conjunction is equivalent to knowledge of every conjunct.
Yes, yes, baba! For at least twenty-five centuries the Jain have been saying that to know one thing fully is to know all things because everything is related. That's why no thing can be known fully or defined in other than Tarskian primitive terms without creating an infinite regress.

Even Hegel- who had some Christian blood in his veins- said something similar. However, that's the reason that Jainism for a good purpose, but also Hegelian shite for a bad one, has a dynamic ontology and a non binary logic.
P is the conjunction of all propositions not-W, where W is a possibility in which not-P.
Not if P is an impredicative proposition of a certain type.  Nor can it be known that P is not impredicative unless everything is defined in terms of Tarskian primitives.

The Nicaraguan horcrux of the neighbour's cat is the conjunction of all horcruxes which are not Nicaraguanly feline and neighbourly to myself. Harry Potter destroyed all the other horcruxes. So let us all sing Hosanna to Chairman Meow!
That suffices to yield an equivalence: S knows that P iff, for every possibility W in which not-P, S knows that not-W.
Telling stupid lies suffices for any purpose answered by telling even less utile stupid lies. The problem here is that if P does not feature primitive terms, or things defined in terms of such primitive notions, then it is 'anything goes.' I have equal warrant for saying P- like every other proposition- affirms a unvocal truth about the Nicaraguan horcrux of my neighbour's cat. You may still cling to your interpretation of P. But, epistemologically speaking, there is no difference between us.

In the real world, we know we live under Knightian uncertainty and that it is an epistemological error- and a big fucking methodological boo boo, not to say recipe for 'weapons of financial mass destruction' - to speak as if we have access to objective probability distributions absent an unitary generator of ergodicity.

Akrebia can have its 'total reflective equilibrium' after some invisible hand has tossed off the (non-local Bell) 'End of History' but Economia must milk that steer to some more utile end. After all, paideia is but vatsalya. Lewis's subject was but a Pedagogy. And that means milking cows so neoteny may be prolonged.
Contraposing and cancelling a double negation: iff every possibility which S does not know not to obtain is one in which P. For short: iff P holds throughout S's epistemic possibilities. Yet to get this far, we need no substantive definition of knowledge at all!
Coz we are on the same footing as votaries of the Nicaraguan horcrux of my neighbour's cat. Knowledge ought not to have a 'substantive definition' for Epistemology-  though it may do so for passing the test (known as 'the Knowledge') to become a black cabbie in London- precisely because it must remain an 'undefined term'- like 'Choice' for Econ or Decision theory- in order to forfend against outright idiocy.
To turn this into a substantive definition, in fact the very definition we gave before, we need to say one more thing: S's epistemic possibilities are  every possibility; hence in every possibility left uneliminated by S's evidence, no matter who S may be and no matter what his evidence may be.
The possibility that Possibility itself is but the purring of my neighbour's cat- amplified through Time and Space by her Nicaraguan horcrux- is, by Lewis's own logic, equivalent to the proposition that all possibilities have a unitary ergodic generator. This means every S must immediately bow down to Chairman Meow!
So, next, we need to say what it means for a possibility to be eliminated or not. Here I say that the uneliminated possibilities are those in which the subject's entire perceptual experience and memory are just as they actually are.
Lewis is assuming that 'perceptual experience and memory' are functionally linked to the set of epistemic possibilities. This need not be true. Some epistemic possibilities may be purely random. Furthermore, epistemic possibilities may be multiply realizable for quite different reasons- e.g. mimetic effects or simple error. If cognitive operations are costly then it is likely that most 'epistemic possibilities' have no functional link to perceptual experience and memory. We don't bother devoting much thought to stuff that scarcely matters to us.
There is one possibility that actually obtains (for the subject and at the time in question); call it actuality.
Why only one? Several possibilities may correspond to one cognitive state.
Then a possibility W is uneliminated iff the subject's perceptual experience and memory in W exactly match his perceptual experience and memory in actuality. (If you want to include other alleged forms of basic evidence, such as the evidence of our extrasensory faculties, or an innate disposition to believe in God, be my guest. If they exist, they should be included. If not, no harm done if we have included them conditionally.) Note well that we do not need the 'pure sense-datum language' and the 'incorrigible protocol statements' that for so long bedevilled foundationalist epistemology.
But we do need a very silly belief that there is a strict functional relationship between something abstract and fundamentally useless like 'epistemic possibilities' and something which evolved on an uncertain fitness landscape- viz. the human brain. What survival value would the human mind gain by displaying this functional relationship? We could easily show that it would be safer not to have it at all so as to baffle a predator or parasite and to execute regret minimizing strategies in a timely manner.
It matters not at all whether there are words to capture the subject's perceptual and memory evidence, nothing more and nothing less.
But this means this doctrine can never be tested.
If there are such words, it matters not at all whether the subject can hit upon them. The given does not consist of basic axioms to serve as premises in subsequent arguments. Rather, it consists of a match between possibilities.
These are not real possibilities but just something invented for the purpose of armchair philosophy. What survival value do they have? If they don't have survival value then one could say that S could be a cat or a rock or a certain shade of purple which briefly appears on a distant planet.

If what you say of a human mind can't be tested and could be said with equal truth of a rock or a hue of purple, then you are talking nonsense.
When perceptual experience E (or memory) eliminates a possibility W, that is not because the propositional content of the experience conflicts with W. (Not even if it is the narrow content.) The propositional content of our experience could, after all, be false. Rather, it is the existence of the experience that conflicts with W: W is a possibility in which the subject is not having experience E. Else we would need to tell some fishy story of how the experience has some sort of infallible, ineffable, purely phenomenal propositional content. .. Who needs that? Let E have propositional content P. Suppose even - something I take to be an open question - that E is, in some sense, fully characterized by P. Then I say that E eliminates W iff W is a possibility in which the subject's experience or memory has content different from P.
Even if the subject is a rock or a shade of purple.
I do not say that E eliminates W iff W is a possibility in which P is false. Maybe not every kind of sense perception yields experience; maybe, for instance, the kinaesthetic sense yields not its own distinctive sort of sense-experience but only spontaneous judgements about the position of one's limbs. If this is true, then the thing to say is that kinaesthetic evidence eliminates all possibilities except those that exactly resemble actuality with respect to the subject's spontaneous kinaesthetic judgements.
Yet we often stumble or suffer other accidents because our kinaesthetic sense is only as good as it has to be to endow some survival value on our fitness landscape.
In saying this, we would treat kinaesthetic evidence more on the model of memory than on the model of more typical senses.
Because memory is notoriously infallible.
Finally, we must attend to the word 'every'. What does it mean to say that every possibility in which not-P is eliminated?
Nothing at all because this statement applies equally to rocks and shades of purple.
An idiom of quantification, like 'every', is normally restricted to some limited domain.
Idioms are normally restricted to utlile domains by sensible people. Failure to do so, results in this sort of nonsense.
If I say that every glass is empty, so it's time for another round, doubtless I and my audience are ignoring most of all the glasses there are in the whole wide world throughout all of time.
Why say every glass is empty? It is enough that some are emptying for it to be wise to order another round. What is important here is that someone order and pay for it.
They are outside the domain. They are irrelevant to the truth of what was said. Likewise, if I say that every uneliminated possibility is one in which P, or words to that effect, I am doubtless ignoring some of all the uneliminated alternative possibilities that there are. They are outside the domain, they are irrelevant to the truth of what was said.
What 'truth' is being spoken when you say something which could equally apply to a rock or a shade of purple?
But, of course, I am not entitled to ignore just any possibility I please. Else true ascriptions of knowledge, whether to myself or to others, would be cheap indeed. I may properly ignore some uneliminated possibilities; I may not properly ignore others. Our definition of knowledge requires a sotto voce proviso. S knows that P iff S's evidence eliminates every possibility in which not-/' - Psst! - except for those possibilities that we are properly ignoring.
Pragmatics does indeed work in this way but it has a pragmatic, utile, end. Imprecision in language does not matter because it is being used in an imperative action guiding way to some mutually beneficial purpose.

Lewis may believe that some useful purpose is served by telling a story about how he has found a workaround for a purely imaginary problem- viz steering between the Scylla of Infallibility with the Charybdis of Skepticism- but both are as mythical as the monsters in Homer.
Unger suggests an instructive parallel.  Just as P is known iff there are no uneliminated possibilities of error, so likewise a surface is flat iff there are no bumps on it. We must add the proviso: Psst! - except for those bumps that we are properly ignoring.
What bumps can be safely ignored is a question for structural engineers or architects or other such experts with an idiographic understanding of what is required or what is at stake.
Else we will conclude, absurdly, that nothing is flat. (Simplify by ignoring departures from flatness that consist of gentle curvature.)
Algebraic geometry is a nomothetic discipline of great utility. This is not the path it takes. Why should philosophy be different? Is it not already considered a worthless discipline? Why provide more ammunition to its critics?
We can restate the definition. Say that we presuppose proposition Q iff we ignore all possibilities in which not-Q. To close the circle: we ignore just those possibilities that falsify our presuppositions. Proper presupposition corresponds, of course, to proper ignoring. Then S knows that P iff S's evidence eliminates every possibility in which notP - Psst! - except for those possibilities that conflict with our proper presuppositions.
Wonderful! We can say that a rock on Mars knows the purple hue of twilight on a planet in a distant galaxy because its evidence eliminates every possibility in which this is not the case. This is thanks to our proper presupposition that Lewis was a smart guy who said smart stuff.

The rest of (modal) epistemology examines the sotto voce proviso. It asks: what may we properly presuppose in our ascriptions of knowledge? Which of all the uneliminated alternative possibilities may not properly be ignored? Which ones are the 'relevant alternatives'? - relevant, that is, to what the subject does and doesn't know.
There is only one proper presupposition when it comes to ascriptions of knowledge viz that it involves a sentient being who makes better choices thanks to that knowledge.

In philosophical terms, this means treating knowledge as a 'primitive notion' which must be left undefined.

In reply, we can list several rules. 1~ We begin with three prohibitions: rules to tell us what possibilities we may not properly ignore. First, there is the Rule of Actuality. The possibility that actually obtains is never properly ignored; actuality is always a relevant alternative; nothing false may properly be presupposed. It follows that only what is true is known, wherefore we did not have to include truth in our definition of knowledge. The rule is 'externalist' - the subject himself may not be able to tell what is properly ignored. In judging which of his ignorings are proper, hence what he knows, we judge his success in knowing - not how well he tried.
Lewis wrote a paper called 'Megethology is Mathematics'. It was shit. Mathematicians aren't saying 'wow, that bloke Lewis sure opened up a lot of new vistas for us.' How the fuck can a bunch of stupid wankers who have made no contribution to Maths or Science apply an 'externalist' rule?

Externalist rules are fine and dandy for an omniscient being. Currently, we don't know whether an omniscient being is 'compossible'. Just as Econ had to get rid of 'externalist theories of value'- like the Marxian one- so too must every other discipline. The thing is garbage.
When the Rule of Actuality tells us that actuality may never be properly ignored,
This is a silly rule. There are all sorts of aspects to 'actuality' which are far beyond our ken. We can and should ignore such aspects till technology improves.
we can ask: whose actuality? Ours, when we ascribe knowledge or ignorance to others? Or the subject's? In simple cases, the question is silly. (In fact, it sounds like the sort of pernicious nonsense we would expect from someone who mixes up what is true with what is believed.) There is just one actual world, we the ascribers live in that world, the subject lives there too, so the subject's actuality is the same as ours.
Lewis is being silly. When we model the behavior of others we use idealized 'representative agents' who don't exist in actuality. The reason he has to pretend that 'Actuality' can be known, and having been known, can be most reprehensibly ignored is because he wouldn't accept that knowledge should be a primitive term. He tried to define it in terms of infallibility and ended up talking nonsense.
But there are other cases, less simple, in which the question makes perfect sense and needs an answer. Someone may or may not know who he is; someone may or may not know what time it is. Therefore I insisted that the propositions that may be known must include propositions de se et nunc; and likewise that the possibilities that may be eliminated or ignored must include possibilities de se et nunc.
A quite common de se et nunc knowledge is that all the other human beings have been killed and replaced by demons or evil robots. Indeed, it may be that all de se et nunc knowledge is radically ontologically dysphoric- there is no world we share in common because Evolution itself has found that ontological dysphoria is regret minimizing and has survival value.
Now we have a good sense in which the subject's actuality may be different from ours. I ask today what Fred knew yesterday. In particular, did he then know who he was? Did he know what day it was? Fred's actuality is the possibility de se et nunc of being Fred on September 19th at such and-such possible world; whereas my actuality is the possibility de se et nunc of being David on September 20th at such-and-such world.
But if so, Fred's actuality is also the possibility de se et nunc of suckling the hypothetical cock of the Nicaraguan horcrux of the neighbor's cat. This the problem with talking high falutin' shite. Others can get in on the act.
So far as the world goes, there is no difference: Fred and I are worldmates, his actual world is the same as mine. But when we build subject and time into the possibilities de se et nunc, then his actuality yesterday does indeed differ from mine today.
Only because Chairman Meow has willed it so. All hail, Meow the Merciless!
 Next, there is the Rule of Belief A possibility that the subject believes to obtain is not properly ignored, whether or not he is right to so believe.
Again, a silly rule. We have inconsistent beliefs. I think Mum's cooking is the best but this belief of mine should be ignored in choosing a restaurant to dine at.
Next, there is the Rule of Resemblance. Suppose one possibility saliently resembles another. Then if one of them may not be properly ignored, neither may the other.
Garbage! A professional Donald Trump impersonator saliently resembles POTUS. However, we can safely ignore the possibility that the leader of the Free World is handing out canapes at our office party.

Why is Lewis writing this shite? Let us see-

It is the Rule of Resemblance that explains why you do not know that you will lose the lottery, no matter what the odds are against you and no matter how sure you should therefore be that you will lose. For every ticket, there is the possibility that it will win. These possibilities are saliently similar to one another: so either every one of them may be properly ignored, or else none may.
Nonsense! The lottery may have been fixed in advance by a crime syndicate. It is silly to have a 'Rule of Resemblance' when we can't know that 'salient similarity' arises from a unitary generator of ergodicity.
The Rule of Resemblance also is the rule that solves the Gettier problems: other cases of justified true belief that are not knowledge. (1) I think that Nogot owns a Ford, because I have seen him driving one; but unbeknownst to me he does not own the Ford he drives, or any other Ford. Unbeknownst to me, Havit does own a Ford, though I have no reason to think so because he never drives it, and in fact I have often seen him taking the tram. My justified true belief is that one of the two owns a Ford.
This is not a justified true belief. Most people lease cars nowadays.
But I do not know it; I am right by accident.
As a person who simply guessed would have been right.
Next, there is the Rule of Reliability. This time, we have a presumptive rule about what may be properly ignored; and it is by means of this rule that we capture what is right about causal or reliabilist theories of knowing. Consider processes whereby information is transmitted to us: perception, memory, and testimony. These processes are fairly reliable.  Within limits, we are entitled to take them for granted. We may properly presuppose that they work without a glitch in the case under consideration. Defeasibly - very defeasibly! - a possibility in which they fail may properly be ignored. 
Why not just ignore all these Rules? They are silly. Lewis has only introduced them to give a semblance of reasoning to an unreasonable project.

Don't you smell a rat? Haven't I, by my own lights, been saying what cannot be said? (Or whistled either.) If the story I told was true, how have I managed to tell it? In trendyspeak, is there not a problem of reflexivity? Does not my story deconstruct itself? I said: S knows that P iff S's evidence eliminates every possibility in which not-PPsst! - except for those possibilities that we are properly ignoring. That 'psst' marks an attempt to do the impossible - to mention that which remains unmentioned. I am sure you managed to make believe that I had succeeded. But I could not have done.
Nobody could believe Lewis succeeded. Most people understand that knowledge is the proper primitive notion for Epistemology. They can't be taken by puerile assertions and a delusive Akrebia featuring a whole bunch of silly rules.

Consider the following-
 A problem due to Saul Kripke turns upon the closure of knowledge under implication. P implies that any evidence against P is misleading. So, by closure, whenever you know that P, you know that any evidence against P is misleading. And if you know that evidence is misleading, you should pay it no heed. Whenever we know - and we know a lot, remember - we should not heed any evidence tending to suggest that we are wrong. But that is absurd. Shall we dodge the conclusion by denying closure? I think not.
This is silly. We all understand that our minds can become 'closed', we get infected by 'group-think', because of our faith in a particular type of knowledge. Thus we need to sit up and pay attention when an experiment falsifies our received wisdom. Since 'knowledge' is a primitive notion, we don't need to say anything about whether or not it is closed under implicature. We just need to junk the old model and find a new one which can lead to new discoveries or innovations.

Lewis ends thus-
In trying to thread a course between the rock of fallibilism and the whirlpool of scepticism, it may well seem as if I have fallen victim to both at once.
No. It seems you refused to accept that knowledge is a primitive notion and so wrote worthless shite featuring definitions of it.
For do I not say that there are all those uneliminated possibilities of error? Yet do I not claim that we know a lot? Yet do I not claim that knowledge is, by definition, infallible knowledge? I did claim all three things. But not all at once! Or if I did claim them all at once, that was an expository shortcut, to be taken with a pinch of salt. To get my message across, I bent the rules. If I tried to whistle what cannot be said, what of it? I relied on the cardinal principle of pragmatics, which overrides every one of the rules I mentioned: interpret the message to make it make sense - to make it consistent, and sensible to say.
The cardinal rule of pragmatics is not to listen when people start shitting higher than their arsehole.
When you have context-dependence, ineffability can be trite and unmysterious. Hush! [moment of silence] I might have liked to say, just then, 'All of us are silent'. It was true. But I could not have said it truly, or whistled it either. For by saying it aloud, or by whistling, I would have rendered it false. I could have said my say fair and square, bending no rules. It would have been tiresome, but it could have been done. The secret would have been to resort to 'semantic ascent'. I could have taken great care to distinguish between (1) the language I use when I talk about knowledge, or whatever, and (2) the second language that I use to talk about  the semantic and pragmatic workings of the first language.
So knowledge would be undefined for Epistemology, though it would have a meta-language in which types of knowledge would be distinguished and analyzed. This would not involve any histrionic talk of the Scylla of Infallibility or Charybdis of Skepticism.
If you want to hear my story told that way, you probably know enough to do the job for yourself. If you can, then my informal presentation has been good enough.
But the story is stupid. If we rewrote it properly, the sensational element would disappear. It would just be a boring piece of hackwork which might get one a gig as a pedagogue enviously teaching s kids who will make ten times your salary within a decade.

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