Suppose that human beings had absolutely no idea how they performed arithmetic.  Imagine that human beings had evolved, rather than having learned, the ability to count sheep and add sheep.  People using this built-in ability have no idea how it worked, the way Aristotle had no idea how his visual cortex supported his ability to see things.  Peano Arithmetic as we know it has not been invented.  There are philosophers working to formalize numerical intuitions, but they employ notations such as

Plus-Of(Seven, Six) = Thirteen

to formalize the intuitively obvious fact that when you add "seven" plus "six", of course you get "thirteen".

In this world, pocket calculators work by storing a giant lookup table of arithmetical facts, entered manually by a team of expert Artificial Arithmeticians, for starting values that range between zero and one hundred.  While these calculators may be helpful in a pragmatic sense, many philosophers argue that they're only simulating addition, rather than really adding.  No machine can really count - that's why humans have to count thirteen sheep before typing "thirteen" into the calculator.  Calculators can recite back stored facts, but they can never know what the statements mean - if you type in "two hundred plus two hundred" the calculator says "Error: Outrange", when it's intuitively obvious, if you know what the words mean, that the answer is "four hundred".

Philosophers, of course, are not so naive as to be taken in by these intuitions.  Numbers are really a purely formal system - the label "thirty-seven" is meaningful, not because of any inherent property of the words themselves, but because the label refers to thirty-seven sheep in the external world.  A number is given this referential property by its semantic network of relations to other numbers.  That's why, in computer programs, the LISP token for "thirty-seven" doesn't need any internal structure - it's only meaningful because of reference and relation, not some computational property of "thirty-seven" itself.

No one has ever developed an Artificial General Arithmetician, though of course there are plenty of domain-specific, narrow Artificial Arithmeticians that work on numbers between "twenty" and "thirty", and so on.  And if you look at how slow progress has been on numbers in the range of "two hundred", then it becomes clear that we're not going to get Artificial General Arithmetic any time soon.  The best experts in the field estimate it will be at least a hundred years before calculators can add as well as a human twelve-year-old.

But not everyone agrees with this estimate, or with merely conventional beliefs about Artificial Arithmetic.  It's common to hear statements such as the following:

  • "It's a framing problem - what 'twenty-one plus' equals depends on whether it's 'plus three' or 'plus four'.  If we can just get enough arithmetical facts stored to cover the common-sense truths that everyone knows, we'll start to see real addition in the network."
  • "But you'll never be able to program in that many arithmetical facts by hiring experts to enter them manually.  What we need is an Artificial Arithmetician that can learn the vast network of relations between numbers that humans acquire during their childhood by observing sets of apples."
  • "No, what we really need is an Artificial Arithmetician that can understand natural language, so that instead of having to be explicitly told that twenty-one plus sixteen equals thirty-seven, it can get the knowledge by exploring the Web."
  • "Frankly, it seems to me that you're just trying to convince yourselves that you can solve the problem.  None of you really know what arithmetic is, so you're floundering around with these generic sorts of arguments.  'We need an AA that can learn X', 'We need an AA that can extract X from the Internet'.  I mean, it sounds good, it sounds like you're making progress, and it's even good for public relations, because everyone thinks they understand the proposed solution - but it doesn't really get you any closer to general addition, as opposed to domain-specific addition.  Probably we will never know the fundamental nature of arithmetic.  The problem is just too hard for humans to solve."
  • "That's why we need to develop a general arithmetician the same way Nature did - evolution."
  • "Top-down approaches have clearly failed to produce arithmetic.  We need a bottom-up approach, some way to make arithmetic emerge.  We have to acknowledge the basic unpredictability of complex systems."
  • "You're all wrong.  Past efforts to create machine arithmetic were futile from the start, because they just didn't have enough computing power.  If you look at how many trillions of synapses there are in the human brain, it's clear that calculators don't have lookup tables anywhere near that large.  We need calculators as powerful as a human brain.  According to Moore's Law, this will occur in the year 2031 on April 27 between 4:00 and 4:30 in the morning."
  • "I believe that machine arithmetic will be developed when researchers scan each neuron of a complete human brain into a computer, so that we can simulate the biological circuitry that performs addition in humans."
  • "I don't think we have to wait to scan a whole brain.  Neural networks are just like the human brain, and you can train them to do things without knowing how they do them.  We'll create programs that will do arithmetic without we, our creators, ever understanding how they do arithmetic."
  • "But Gödel's Theorem shows that no formal system can ever capture the basic properties of arithmetic.  Classical physics is formalizable, so to add two and two, the brain must take advantage of quantum physics."
  • "Hey, if human arithmetic were simple enough that we could reproduce it in a computer, we wouldn't be able to count high enough to build computers."
  • "Haven't you heard of John Searle's Chinese Calculator Experiment?  Even if you did have a huge set of rules that would let you add 'twenty-one' and 'sixteen', just imagine translating all the words into Chinese, and you can see that there's no genuine addition going on.  There are no real numbers anywhere in the system, just labels that humans use for numbers..."

There is more than one moral to this parable, and I have told it with different morals in different contexts.  It illustrates the idea of levels of organization, for example - a CPU can add two large numbers because the numbers aren't black-box opaque objects, they're ordered structures of 32 bits.

But for purposes of overcoming bias, let us draw two morals:

  • First, the danger of believing assertions you can't regenerate from your own knowledge.
  • Second, the danger of trying to dance around basic confusions.

Lest anyone accuse me of generalizing from fictional evidence, both lessons may be drawn from the real history of Artificial Intelligence as well.

The first danger is the object-level problem that the AA devices ran into: they functioned as tape recorders playing back "knowledge" generated from outside the system, using a process they couldn't capture internally.  A human could tell the AA device that "twenty-one plus sixteen equals thirty-seven", and the AA devices could record this sentence and play it back, or even pattern-match "twenty-one plus sixteen" to output "thirty-seven!", but the AA devices couldn't generate such knowledge for themselves.

Which is strongly reminiscent of believing a physicist who tells you "Light is waves", recording the fascinating words and playing them back when someone asks "What is light made of?", without being able to generate the knowledge for yourself.  More on this theme tomorrow.

The second moral is the meta-level danger that consumed the Artificial Arithmetic researchers and opinionated bystanders - the danger of dancing around confusing gaps in your knowledge.  The tendency to do just about anything except grit your teeth and buckle down and fill in the damn gap.

Whether you say, "It is emergent!", or whether you say, "It is unknowable!", in neither case are you acknowledging that there is a basic insight required which is possessable, but unpossessed by you.

How can you know when you'll have a new basic insight?  And there's no way to get one except by banging your head against the problem, learning everything you can about it, studying it from as many angles as possible, perhaps for years.  It's not a pursuit that academia is set up to permit, when you need to publish at least one paper per month.  It's certainly not something that venture capitalists will fund.  You want to either go ahead and build the system now, or give up and do something else instead.

Look at the comments above: none are aimed at setting out on a quest for the missing insight which would make numbers no longer mysterious, make "twenty-seven" more than a black box.  None of the commenters realized that their difficulties arose from ignorance or confusion in their own minds, rather than an inherent property of arithmetic.  They were not trying to achieve a state where the confusing thing ceased to be confusing.

If you read Judea Pearl's "Probabilistic Reasoning in Intelligent Systems: Networks of Plausible Inference" then you will see that the basic insight behind graphical models is indispensable to problems that require it.  (It's not something that fits on a T-Shirt, I'm afraid, so you'll have to go and read the book yourself.  I haven't seen any online popularizations of Bayesian networks that adequately convey the reasons behind the principles, or the importance of the math being exactly the way it is, but Pearl's book is wonderful.)  There were once dozens of "non-monotonic logics" awkwardly trying to capture intuitions such as "If my burglar alarm goes off, there was probably a burglar, but if I then learn that there was a small earthquake near my home, there was probably not a burglar."  With the graphical-model insight in hand, you can give a mathematical explanation of exactly why first-order logic has the wrong properties for the job, and express the correct solution in a compact way that captures all the common-sense details in one elegant swoop.  Until you have that insight, you'll go on patching the logic here, patching it there, adding more and more hacks to force it into correspondence with everything that seems "obviously true".

You won't know the Artificial Arithmetic problem is unsolvable without its key.  If you don't know the rules, you don't know the rule that says you need to know the rules to do anything.  And so there will be all sorts of clever ideas that seem like they might work, like building an Artificial Arithmetician that can read natural language and download millions of arithmetical assertions from the Internet.

And yet somehow the clever ideas never work.  Somehow it always turns out that you "couldn't see any reason it wouldn't work" because you were ignorant of the obstacles, not because no obstacles existed.  Like shooting blindfolded at a distant target - you can fire blind shot after blind shot, crying, "You can't prove to me that I won't hit the center!"  But until you take off the blindfold, you're not even in the aiming game.  When "no one can prove to you" that your precious idea isn't right, it means you don't have enough information to strike a small target in a vast answer spaceUntil you know your idea will work, it won't.

From the history of previous key insights in Artificial Intelligence, and the grand messes which were proposed prior to those insights, I derive an important real-life lesson:  When the basic problem is your ignorance, clever strategies for bypassing your ignorance lead to shooting yourself in the foot.


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Well, shooting randomly at a distant target is more likely to produce a bulls-eye than not shooting at all, even though you're almost certainly going to miss (and probably shoot yourself in the foot while you're at it). It's probably better to try to find a way to take off that blindfold. As you suggest, we don't yet understand intelligence, so there's no way we're going to make an intelligent machine without either significantly improving our understanding or winning the proverbial lottery.

"Programming is the art of figuring out what you want so precisely that even a machine can do it." - Some guy who isn't famous

Well shooting randomly is perhaps a bad idea, but I think the best we can do is shoot systematically, which is hardly better (takes exponentially many bullets). So you either have to be lucky, or hope the target isn't very far, so you don't need to a wide cone to take pot shots at, or hope P=NP.

1[anonymous]9yquadratically many, actually. EDIT: well, in the case of actual shooting at least.

@Doug & Gray: AGI is a William Tell target. A near miss could be very unfortunate. We can't responsibly take a proper shot till we have an appropriate level of understanding and confidence of accuracy.

0[anonymous]10yThis keeps on coming up, is there somewhere this is explained in detail? Also, have possible solutions been looked at such as constructing the AI in a controlled environment? If so why wouldn't any of them work work? Thanks to whoever responds.


Did you include your own answer to the question of why AI hasn't arrived yet in the list? :-)

This is a nice post. Another way of stating the moral might be: "If you want to understand something, you have to stare your confusion right in the face; don't look away for a second."

So, what is confusing about intelligence? That question is problematic: a better one might be "what isn't confusing about intelligence?"

Here's one thing I've pondered at some length. The VC theory states that in order to generalize well a learning machine m... (read more)

3JohnH10yThis is what some philosophers have purposed, others have thought we start as a blank slate. The research into the subject has shown that babies do start with some sort of working model of things. That is we begin life with a set of preset preferences and the ability to distinguish those preferences and a basic understanding of geometric shapes.
5wedrifid10yIt would be shocking if we didn't have preset functions. Calves, for example, can walk almost straight away and swim not much longer. We aren't going to entirely eliminate the mammalian ability to start with a set of preset features there just isn't enough pressure to keep a few of them.
6Cyan10yIf you put a newborn whose mother had an unmedicated labor on the mother's stomach, the baby will move up to a breast and start to feed.
2wedrifid10yGood point. Drink (food), breathe, scream and a couple of cute reactions to keep caretakers interested. All you need to bootstrap a human growth process. There seems to be something built in about eye contact management too - because a lack there is an early indicator that something is wrong.
2Houshalter7yNot terribly relevant to your point, but it's likely human sense of cuteness is based on what babies do rather than the other way around.
1Nornagest7yI'd replace "human" with "mammalian" -- most young mammals share a similar set of traits, even those that aren't constrained as we are by big brains and a pelvic girdle adapted to walking upright. That seems to suggest a more basal cuteness response; I believe the biology term is "baby schema". Other than that, yeah.
4Eugene9yConversely, studies with newborn mammals have shown that if you deprive them of something as simple as horizontal lines, they will grow up unable to distinguish lines that approach 'horizontalness'. So even separating the most basic evolved behavior from the most basic learned behavior is not intuitive.
0Cyan9yThe deprivation you're talking about takes place over the course of days and weeks -- it reflects the effects of (lack of) reinforcement learning, so it's not really germane to a discussion of preset functions that manifest in the first few minutes after birth.
3Eugene8yIt's relevant insofar as we shouldn't make assumptions on what is and is not preset simply based on observations that take place in a "typical" environment.
2Cyan8yAh, a negative example. Fair point. Guess I wasn't paying enough attention and missed the signal you meant to send by using "conversely" as the first word of your comment.
1Eugene8yThat was lazy of me, in retrospect. I find that often I'm poorer at communicating my intent than I assume I am.
1Kenny8yIllusion of transparency [] strikes again!
0Houshalter6yArtificial Neural Networks have been trained with millions of parameters. There are a lot of different methods of regularization like dropconnect or sparsity constraints. But the brain does online learning. Overfitting isn't as big of a concern because it doesn't see the data more than once.
0Punoxysm6yOn the other hand, architecture matters. The most successful neural network for a given task has connections designed for the structure of that task, so that it will learn much more quickly than a fully-connected or arbitrarily connected network. The human brain appears to have a great deal of information and structure in its architecture right off the bat.
0[anonymous]6yThe human brain appears to engage in hierarchical learning, which is what allows it to leverage huge amounts of "general case" abstract knowledge in attacking novel specific problems put before it.
0Houshalter6yI'm not saying that you're wrong, but the state of the art in computer vision is weight sharing which biological NNs probably can't do. Hyper parameters like the number of layers and how local the connections should be, are important but they don't give that much prior information about the task. I may be completely wrong, but I do suspect that biological NNs are far more general purpose and less "pre-programmed" than is usually thought. The learning rules for a neural network are far simpler than the functions they learn. Training neural networks with genetic algorithms is extremely slow.
0Punoxysm6yArchitecture of the V1 and V2 areas of the brain, which Convolutional Neural Networks and other ANNs for vision borrow heavily from, is highly geared towards vision, and includes basic filters that detect stripes, dots, corners, etc. that appear in all sorts of computer vision work. Yes, no backpropagation or weight-sharing is directly responsible for this, but the presence of local filters is still what I would call very specific architecture (I've studied computer vision and inspiration it draws from early vision specifically, so I can say more about this). The way genetic algorithms tune weights in an ANN (and yes, this is an awful way to train an ANN) is very different from the way they work in actually evolving a brain; working on the genetic code that develops the brain. I'd say they are so wildly different that no conclusions from the first can be applied to the second. During a single individual's life, Hebbian and other learning mechanisms in the brain are distinct from gradient learning, but can achieve somewhat similar things.

That's not how William Tell managed it. He had to practice aiming at less-dangerous targets until he became an expert, and only then did he attempt to shoot the apple.

It is not clear to me that it is desirable to prejudge what an artificial intelligence should desire or conclude, or even possible to purposefully put real constraints on it in the first place. We should simply create the god, then acknowledge the truth: that we aren't capable of evaluating the thinking of gods.

1wizzwizz49moBut it shouldn't conclude that throwing large asteroids at Yellowstone is a good idea, nor desire to do it. If you follow this strategy, you'll doom us. Simple as that.

Adding to DanBurFoot, is there a link you want to point to that shows your real, tangible results for AI, based on your superior methodology?

For what it's worth, Benoit Essiambre, the things you have just said are nonsense. The reason logicians seem to be unable to make a distinction between 1.999... and 2 is that there is no distinction. They are not two different definable real numbers, they are the same definable real number.

-3taryneast10yExcept that 1.9999... < 2 Edit: here's the proof that I'm wrong mathematically (from the provided Wikipedia link): "Multiplication of 9 times 1 produces 9 in each digit, so 9 × 0.111... equals 0.999... and 9 × 1⁄9 equals 1, so 0.999... = 1"
7Paul Crowley10yNo, these are two different ways of writing the same number [].
2taryneast10yOk. Interesting. I can see and agree that 0.999... can in the limit equal two, whereas in any finite representation would still be less than 2. I don't consider them to be "the same number" in that sense... even though they algebraically equate (once the limit is reached) in a theoretical framework that can encompass infinities. ie, in maths, I'd equate them but in the "real world" - I'd treat them separately. Edit: and reading further... it seems I'm wrong again. Of course, the whole point of putting "..." is to represent the fact that this is the limit of the decimal expansion of 0.999... to infinity. therefore yep, 1.999... = 2 Where my understanding failed me is that 1.999... does not in fact represent the summation of the infinite set of 1 + 0.9 + 0.09 + ... which summation could, in fact, simply not be taken to its full limit. The representation "1.999..." can only represent either the set or the limit of the set, and mathematical convention has it as the latter, not the former.
2byrnema10yAnother argument that may be more convincing on a gut level: 9x(1/9) is exactly equal to 1, correct? Find the decimal representation of 1/9 using long division: 1/9=0.11111111... (note there is no different or superior way to represent this number as a decimal) 9x(1/9) = 9x(0.11111111...)=0.9999999... which we already agreed was exactly equal to 1.
3taryneast10yYes :) See my previous (edited) comment above.
0byrnema10yOh sorry, my bad. I should have read the thread. Or the link.
2taryneast10yNo problem. It is a great proof (there aren't many so simple and succinct). Just bad luck on timing ;)
1Sniffnoy10yNote also that it has to denote the limit, because we want it to denote a number, and the other object you describe (a sequence rather than a set, strictly speaking) isn't a number, just, well, a sequence of numbers.
-1taryneast10yThis is the part I take issue with. It does not have to denote a number, but we choose to let it denote a number (rather than a sequence) because that is how mathematicians find it most convenient to use that particular representation. That sequence is also quite useful mathematically - just not as useful as the number-that-represents-the-limit. Many sequences are considered to be useful... though generally not in algebra - it's more common in Calculus, where such sequences are extremely useful. In fact I'd say that in calculus "just a sequence" is perhaps even more useful than "just a number". My first impression (and thus what I originally got wrong) was that 1.999... represented the sequence and not the limit because, really, if you meant 2, why not just say 2? :)
0JoshuaZ10yWe want it to denote a number for simple consistency. .11111... is a number. It is a limit. 3.14159... should denote a number. Why should 1.99999?... Be any different? If we are going to be at all consistent in our notation they should all represent the same sort of series. Otherwise this is extremely irregular notation to no end.
-1taryneast10yYes, I totally agree with you: consistency and convenience are why we have chosen to use 1.9999... notation to represent the limit, rather than the sequence. consistency and convenience tends to drive most mathematical notational choices (with occasional other influences), for reasons that should be extremely obvious. It just so happened that, o this occasion, I was not aware enough of either the actual convention, or of other "things that this notation would be consistent with" before I guessed at the meaning of this particular item of notation. And so my guessed meaning was one of the two things that I thought would be "likely meanings" for the notation. In this case, my guess was for the wrong one of the two. I seem to be getting a lot of comments that are implying that I should have somehow naturally realised which of the two meanings was "correct"... and have tried very hard to explain why it is not obvious, and not somehow inevitable. Both of my possible interpretations were potentially valid, and I'd like to insist that the sequence-one is wrong only by convention (ie maths has to pick one or the other meaning... it happens to be the most convenient for mathematicians, which happens in this case to be the limit-interpretation)... but as is clearly evidenced by the fact that there is so much confusion around the subject (ref the wikipedia page) - it is not obvious intuitively that one is "correct" and one is "not correct". I maintain that without knowledge of the convention, you cannot know which is the "correct" interpretation. Any assumption otherwise is simply hindsight bias.
2wizzwizz49moThere is no inherent meaning to a set of symbols scrawled on paper. There is no "correct" and "incorrect" way of interpreting it; only convention (unless your goal is to communicate with others). There is no Platonic Ideal of Mathematical Notation, so obviously there is no objective way to pluck the "correct" interpretation of some symbols out of the interstellar void. You are right in as far as you say that. However, you are expected to know the meaning of the notation you use in exactly the same way that you are expected to know the meaning of the words you use. Not knowing is understandable, but observing that it is possible to not-know a convention is not a particular philosophical insight.
0Sniffnoy10yOK, let me put it this way: If we are considering the question "Is 1.999...=2?", the context makes it clear that we must be considering the left hand side as a number, because the RHS is a number. (Would you interpret 2 in that context as the constant 2 sequence? Well then of course they're not equal, but this is obvious and unenlightening.) Why would you compare a number for equality against a sequence? They're entirely different sorts of objects.
-2taryneast10yis "x-squared = 2" ? is a perfectly valid question to ask in mathematics even though the LHS is not obviously an number In this case, it is a formula that can equate to a number... just as the sequence is a (very limited) formula that can equate to 2 - if we take the sequence to its limit; or that falls just shy of 2 - if we try and represent it in any finite/limited way. In stating that 1.9999... is a number, you are assuming the usage of the limit/number, rather than the other potential usage ie, you are falling into the same assumption-trap that I fell into... It's just that your assumption happens to be the one that matches with common usage, whereas mine wasn't ;) Using 1.9999. to represent the limit of the sequence (ie the number) is certainly true by convention (ie "by definition"), but is no means the only way to interpret the symbols. It could just as easily represent the sequence itself... we just don't happen to do that - we define what mathematical symbols refer to... they're just the word/pointers to what we're talking about yes?
2Sniffnoy10yEr... yes it is? In that context, x^2 is a number. We just don't know what number it might be. By contrast, the sequence (1, 1.9, 1.99, ...) is not a number at all. Furthermore, even if we insist on regarding x^2 as a formula with a free variable, your analogy doesn't hold. The sequence (1, 1.9, 1.99, ...) has no free variables; it's one specific sequence. You are correct that the convention could have been that 1.999... represents the sequence... but as I stated before, in that case, the question of whether it equals 2 would not be very meaningful. Given the context you can deduce that we are using the convention that it designates a number.
-3taryneast10yyes I agree, a sequence is not a number, it's sequence... though I wonder if we're getting confused, because we're talking about the sequence, instead of the infinite series (1 + 0.9 + 0.09 +...) which is actually what I had in my head when I was first thinking about 1.999... Along the way, somebody said "sequence" and that's the word I started using... when really I've been thinking about the infinite series.... anyway The infinite series has far less freedom than x^2, but that doesn't mean that it's a different thing entirely from x^2. Lets consider "x - 1" "x -1 " is not a number, until we equate it to something that lets us determine what x is... If we use: "x -1 =4 " however. We can solve-for-x and there are no degrees of freedom. If we use "1.9 < x -1 < 2" we have some minor degree of freedom... and only just a few more than the infinite series in question. Admittedly, the only degree of freedom left to 1.9999... (the series) is to either be 2 or an infinitesimal away from 2. But I don't think that makes it different in kind to x -1 = 4 anyway - I think we're probably just in "violent agreement" (as a friend of mine once used to say) ;) All the bits that I was trying to really say we agree over... now we're just discussing the related maths ;) Ok, lets move into hypothetical land and pretend that 1.9999... represents what I originally though it represents. The comparison with the number 2 provides the meaning that what you want to do is to evaluate the series at its limit. It's totally supportable for you to equate 1.9999... = 2 and determine that this is a statement that is: 1) true when the infinite series has been evaluated to the limit 2) false when it is represented in any finite/limited way Edit: ah... that's why you can't use stars for to-the-power-of ;)
1Sniffnoy10yEr, no... there still seems to be quite a bit of confusion here... Well, if you really think that's not significant... :P It's not clear to me what distinction you're drawing here. A series is a sequence, just written differently. It's not at all clear to me what notion of "degrees of freedom" you're using here. The sequence is an entirely different sort of thing than x^2, in that one is a sequence, a complete mathematical object, while the other is an expression with a free variable. If by "degrees of freedom" you mean something like "free variables", then the sequence has none. Now it's true that, being a sequence of real numbers, it is a function from N to R, but there's quite a difference between the expression 2-10^(-n), and the function (i.e. sequence) n |-> 2-10^(-n) ; yes, normally we simply write the latter as the former when the meaning is understood, but under the hood they're quite different. In a sense, functions are mathematical, expressions are metamathematical. When I say "x^2 is a number", what I mean is essentially, if we're working under a type system, then it has the type "real number". It's an expression with one free variable, but it has type "real number". By contrast, the function x |-> x^2 has type "function from reals to reals", the sequence (1, 1.9, 1.99, ...) has type "sequence of reals"... (I realize that in standard mathematics we don't actually technically work under a type system, but for practical purposes it's a good way to think, and it's I'm pretty sure it's possible to sensibly formulate things this way.) To equate a sequence to a number may technically in a sense return "false", but it's better to think of it as returning "type error". By contrast, equating x^2 to 2 - not equating the function x|->x^2 to 2, which is a type error! - allows us to infer that x^2 is also a number. Note, BTW, that the real numbers don't have any infinitesimals (save for 0, if you count it). Sorry, what does it even mean for it to be "represente
-1taryneast10yah - then I apologise. I need to clarify. I see that there are several points where you've pointed out that I am using mathematical language in a sloppy fashion. How about I get those out of the way first. I should not have used the word "infinitesimal" - as I really meant "a very small number" and was being lazy. I am aware that "the theory of infinitesimals" has an actual mathematical meaning... but this is not the way in which I was using the word. I'll explain what I meant in a bit.. If I write a program that starts by adding 1 to 0.9 then I put it into a loop where it then adds "one tenth of the previous number you just added"... If at any point I tell the program "stop now and print out what you've got so far"... then what it will print out is something that is "a very small number" less than 2. If I left the program running for literally an infinite amount of time, it would eventually reach two. If I stop at any point at all (ie the program is finite), then it will return a number that a very small amount less than two. In this way, the program has generated a finite approximation of 1.999... that is != 2 As humans, we can think about the problem in a way that a stupid computer algorithm cannot, and can prove to ourselves that 1+(0.111.. * 9) actually == 2 exactly. but that is knowledge outside of the proposed "finite" solution/system as described above. Thus the two are different "representations" of 1.999... I am reminded of the old engineering adage that "3 is a good approximation of Pi for all practical purposes" - which tends to make some mathematicians squirm. x^2 has one degree of freedom. x can be any real number 1 < x < 1.1 has less freedom than that. It can be any real number between 1 and 1.1 With the previous description I've given of the difference between the results of a "finite" and "infinite" calculation of the limit of 1.999... (the series), "x = 1.999..." can be either 2 (if we can go to the limit or can think about it in a way o
2[anonymous]10yIf we wanted to talk about the sequence we would never denote it 1.999... We would write {1, 1.9, 1.99, 1.999, ...} and perhaps give the formula for the Nth term, which is 2 - 10^-N.
-1taryneast10yHi Misha, I might also turn that argument back on you and repeat what I said before: "if you meant 2, why not just say 2?" It's as valid as "if you meant the sequence, why not just write {1, 1.9, 1.99, 1.999, ...}"? Clearly there are other reasons for using something that is not the usual convention. There are definitely good reasons for representing infinite series or sequences... as you have pointed out. However - there is no particular reason why mathematics has chosen to use 1.999... to mean the limit, as opposed to the actual infinite series. Either one could be equally validly used in this situation. It is only by common convention that mathematics uses it to represent the actual limit (as n tends to infinity) instead of the other possibility - which would be "the actual limit as n tends to infinity... if we actually take it to infinity, or an infinitesimal less than the limit if we don't", which is how I assumed (incorrectly) that it was to be used However, the other thing you say that "we never denote it 1.999..." pulls out an interesting though, and if I grasp what you're saying correctly, then I disagree with you. As I've mentioned in another comment now - mathematical symbolic conventions are the same as "words" - they are map, not territory. We define them to mean what we want them to mean. We choose what they mean by common consensus (motivated by convenience). It is a very good idea to follow that convention - which is why I decided I was wrong to use it the way I originally assumed it was being used... and from now on, I will use the usual convention... However, you seem to be saying that you think the current way is "the one true way" and that the other way is not valid at all... ie that "we would never denote it 1.9999..." as being some sort of basis of fact out there in reality, when really it's just a convention that we've chosen, and is therefore non-obvious from looking at the symbol without the prior knowledge of the convention (as I did)
1[anonymous]10yYou have a point. I tend to dislike arguments about mathematics that start with "well, this definition is just a choice" because they don't capture any substance about any actual math. As a result, I tried to head that off by (perhaps poorly) making a case for why this definition is a reasonable choice. In any case, I misunderstood the nature of what you were saying about the convention, so I don't think we're in any actual disagreement. If I meant 2, I would say 2. However, our system of writing repeating decimals also allows us to (redundantly) write the repeating decimal 1.999... which is equivalent to 2. It's not a very useful repeating decimal, but it sometimes comes out as a result of an algorithm: e.g. when you multiply 2/9 = 0.222... by 9, you will get 1.999... as you calculate it, instead of getting 2 straight off the bat.
1taryneast10yMe too! Especially as I've just been reading that sequence here about "proving by definition" and "I can define it any way I like"... that's why I tried to make it very clear I wasn't saying that... I also needed to head of the heading off ;) Anyway - I believe we are just in violent agreement here, so no problems ;)
2Jonni9yThe easiest example I've come across is: If (1 ÷ 3 = 0.333...) and (0.999... ÷ 3 = 0.333...) then (1 = 0.999...).
-2Houshalter7yFor what it's worth (and why do I have to pay karma to reply to this comment, I don't get it) there is an infinitesimal difference between the two. An infinitesimal is just like infinity in that it's not a real number. For all practical purposes it is equal to zero, but just like infinity, it has useful mathematical purposes in that it isn't exactly equal to zero. You could plug an infinitesimal into an equation to show how close you can get to zero without actually getting there. If you just replaced it with zero the equation could come out undefined or something. Likewise using 1.999... because of the property that it isn't exactly equal to 2 but is practically equal to 2, could be useful.
4ialdabaoth7yer... I'm not sure if this is the right way to look at it. 1.999999... is 2. Exactly 2. The thing is, there is an infinitesimal difference between '2' and '2'. 1.999999.... isn't "Two minus epsilon", it's "The limit of two minus epsilon as epsilon approaches zero", which is two. EDIT: And to explain the following objection: Yes, absolutely. That's part of the point of infinity. One way of looking at certain kinds of infinity (note that there are several kinds of infinity) is that infinity is one of our placeholders for where rules break down.
1Houshalter7yThis is one of those things that isn't worth arguing over at all, but I will anyways because I'm interested. I'm probably wrong because people much smarter than me have thought about this before, but this still doesn't make any sense to me at all. 1.9 is just 2 minus 0.1, right? And 1.99 is just 2 minus 0.01. Each time you add another 9, you are dividing the number you are subtracting by 10. No matter how many times you divide 0.1 by ten, you will never exactly reach zero. And if it's not exactly zero, then two minus the number isn't exactly two. Even if you do it 3^^^3 times, it will still be more than zero. Weird things happen when you apply infinity, but can it really change a rule that is true for all finite numbers? You can say it approaches 2 but that's not the same as it ever actually reaching it. Does this make any sense?
0[anonymous]10yInteresting... three down-votes but only one soul kind enough to point out why what I said was wrong (thank you ciphergoth). I find that quite disappointing - especially as I've seen some deliberate troll-baiting receive fewer down-votes.
Me: AGI is a William Tell target. A near miss could be very unfortunate. We can't responsibly take a proper shot till we have an appropriate level of understanding and confidence of accuracy.
Caledonian: That's not how William Tell managed it. He had to practice aiming at less-dangerous targets until he became an expert, and only then did he attempt to shoot the apple.

Yes, by "take a proper shot" I meant shooting at the proper target with proper shots. And yes, practice on less-dangerous targets is necessary, but it's not sufficient.

It is no
... (read more)

No, by 2 I mean 1.999...


1.9999... = 2 is not an "issue" or a "paradox" in mathematics.

If you use a limited number of digits in your calculations, then your quantization errors can accumulate. (And suppose the quantity you are measuring is the difference of two much larger numbers.)

Of course it's possible that there's nothing in the real world that corresponds exactly to our so-called "real numbers". But until we actually know what smaller-scale structure it is that we're approximating, it would be crazy to pick some arbitrary "lower-resolution&q... (read more)

"...mathematics that represent continuous scales which would be best represented by the real numbers system with the limited significant digits."

If you limit the number of significant digits, your mathematics are discrete, not continuous. I'm guessing the concept you're really after is the idea of computable numbers. The set of computable numbers is a dense countable subset of the reals.

With the graphical-network insight in hand, you can give a mathematical explanation of exactly why first-order logic has the wrong properties for the job, and express the correct solution in a compact way that captures all the common-sense details in one elegant swoop.

Consider the following example, from Menzies's "Causal Models, Token Causation, and Processes"[*]:

An assassin puts poison in the king's coffee. The bodyguard responds by pouring an antidote in the king's coffee. If the bodyguard had not put the antidote in the coffee, the king would... (read more)

9Eliezer Yudkowsky11yUm, this sounds not correct. The assassin causes the bodyguard to add the antidote; if the bodyguard hadn't seen the assassin do it, he wouldn't have so added. So if you compute the counterfactual the Pearlian way, manipulating the assassin changes the bodyguard's action as well, since the bodyguard causally descends from the assassin.
6SilasBarta11yRight -- and according to Pearl's causal beam method, you would first note that the guard sustains the coffee's (non)deadliness-state against the assassin's action, which ultimately makes you deem the guard the cause of the king's survival.
5thomblake7yFurthermore, if you draw the graph the way Neel seems to suggest, then the bodyguard is adding the antidote without dependence on the actions of the assassin, and so there is no longer any reason to call one "assassin" and the other "bodyguard", or one "poison" and the other "antidote". The bodyguard in that model is trying to kill the king as much as the assassin is, and the assassin's timely intervention saved the king as much as the bodyguard's.

"But until we actually know what smaller-scale structure".

From "Combined, these two theories imply that it is impossible to measure position to a precision greater than the Planck length, or duration to a precision greater than the time a photon traveling at c would take to travel a Planck length"

Therefore, one could in fact say that all time- and distance- derived measurements can in fact be truncated to a fixed number of decimal places without losing any real precision, by using precisions b... (read more)

"When the basic problem is your ignorance, clever strategies for bypassing your ignorance lead to shooting yourself in the foot."

I like this lesson. It rings true to me, but the problem of ego is not one to be overlooked. People like feeling smart and having the status of being a "learned" individual. It takes a lot of courage to profess ignorance in today's academic climate. We are taught that we have such sophisticated techniques to solve really hard problems. There are armies of scientists and engineers working to advance our so... (read more)

anonymous--I'd like to second that motion

I read a book on the philosophy of set theory -- and I get lost right at the point where classical infinite thought was replaced by modern infinite thought. IIRC the problem was paradoxes based on infinite recursion (Zeno et. all) and finding mathematical foundations to satisfy calculus limits. Then something about Cantor, cardinality and some hand wavy 'infinite sets are real!'.

1.999... is just an infinite set summation of finite numbers 1 + 0.9 + 0.09 + ...

Now, how an infinite process on an infinite set can equal an integer is a problem I still grapple... (read more)

-3taryneast10y1.999... does not equal 2 - it just tends towards 2 For all practical purposes, you could substitute one for the other. But in theory, you know that 1.9999... is always just below 2, even though it creeps ever closer. If we ever found a way to magickally "reach infinity" they would finally meet... and be "equal". Edit: The numbers are always going to be slightly different in a finite-space, but equate to the same thing when you allow infinities. ie mathematically, in the limit, they equate to the same value, but in any finite representation, they are different. Further Edit: According to mathematical convention, the notation "1.999..." does refer to the limit. therefore, "1.999..." strictly refers to 2 (not to any finite case that is slightly less than two).
2DanielLC9yIt was nonsense in classical theory. Infinite sum has its own separate definition. There are times in modern mathematics that infinite numbers are used. This is not one of them. I doubt I'm the best at explaining what limits are, so I won't bother. I may be able to tell you what they aren't. They give results similar to the intuitive idea of infinite numbers, but they don't do it in the most intuitively obvious way. They don't use infinite numbers. They use a certain property that at most one number will have in relation to a sequence. In the case of 1, 1.9, 1.99, ..., this number is two. In the case of 1, 0, 1, 0, ..., there is no such number, so the series is said not to converge. No. The question is "Can we make a sensible theoretical way to interpret the numeral 1.999..., that approximately matches our intuitions?" It wasn't easy, but we managed it.
Question: What does this tell you about current methods for measuring intelligence?

Better question: why do you insist that those examples are of failures to acknowledge intelligence when you also insist that we are unable to meaningfully define intelligence?

mclaren, your comment is way too long. I have truncated it and emailed you the full version. Feel free to post the comment to your blog, then post a link to the blog here.

Anonymous (re Planck scales etc.), sure you can truncate your representations of lengths at the Planck length, and likewise for your representations of times, but this doesn't simplify your number system unless you have acceptable ways of truncating all the other numbers you need to use. And, at present, we don't. Sure, maybe really the universe is best considered as some sort of discrete network with some funky structure on it, but that doesn't give us any way of simplifying (or making more appropriate) our mathematics until we know just what sort of disc... (read more)

Thanks g for the tip about computable numbers, that's pretty much what I had in mind. I didn't quite get from the wikipedia article if these numbers could or could not replace the reals for all of useful mathematics but it's interesting indeed.

James, I share your feelings of uneasiness about infinite digits, as you said, the problem is not that these numbers will not represent the same points at the limit but that they shouldn't be taken to the limit so readily as this doesn't seem to add anything to mathematics but confusion.


If I recall my Newton correctly, the only way to take this "sum of an infinite series" business consistently is to interpret it as shorthand for the limit of an infinite series. (Cf. Newton's Principia Mathematica, Lemma 2. The infinitesimally wide parallelograms are dubitably real, but the area under the curve between the sets of parallelograms is clearly a real, definite area.)


Why shouldn't we take 1.9999... as just another, needlessly complicated (if there's no justifying context) way of writing "2"? Just as I could conceivably count "1, 2, 3, 4, d(5x)/dx, 6, 7" if I were a crazy person.

Benquo, I see two possible reasons:

1) '2' leads to confusion as to whether we are representing a real or a natural number. That is, whether we are counting discrete items or we are representing a value on a continuum. If we are counting items then '2' is correct.

2) If it is clear that we are representing numbers on a continuum, I could see the number of significant digits used as an indication of the amount of uncertainty in the value. For any real problem there is always uncertainty caused by A) the measuring instrument and B) the representation system it... (read more)

Benoit Essiambre,

Right now Wikipedia's article is claiming that calculus cannot be done with computable numbers, but a Google search turned up a paper from 1968 which claims that differentiation and integration can be performed on functions in the field of computable numbers. I'll go and fix Wikipedia, I suppose.

Benoit Essiambre,

You say:

"1) '2' leads to confusion as to whether we are representing a real or a natural number. That is, whether we are counting discrete items or we are representing a value on a continuum."

If I recall correctly, this "confusion" is what allowed modern, atomic chemistry. Chemical substances -- measured as continuous quantities -- seem to combine in simple natural-number ratios. This was the primary evidence for the existence of atoms.

What is the practical negative consequence of the confusion you're trying to avoid?... (read more)

Benoit, it was "Cyan" and not me who mentioned computable numbers.

Benoit, you assert that our use of real numbers leads to confusion and paradox. Please point to that confusion and paradox.

Also, how would your proposed number system represent pi and e? Or do you think we don't need pi and e?

Well, for example, the fact that two different real represent the same point. 2.00... 1.99... , the fact that they are not computable in a finite amount of time. pi and e are quite representable within a computable number system otherwise we couldn't reliably use pi and e on computers!

Benoit, those are two different ways of writing the same real, just like 0.333... and 1/3 (or 1.0/3.0, if you insist) are the same number. That's not a paradox. 2 is a computable number, and thus so are 2.000... and 1.999..., even though you can't write down those ways of expressing them in a finite amount of time. See the definition of a computable number if you're confused.

1.999... = 2.000... = 2. Period.


In the decimal numeral system, every number with a terminating decimal representation also has a non-terminating one that ends with recurring nines. Hence, 1.999... = 2, 0.74999... = 0.75, 0.986232999... = 0.986233, etc. This isn't a paradox, and it has nothing to do with the precision with which we measure actual real things. This sort of recurring representation happens in any positional numeral system.

You seem very confused as to the distinction between what numbers are and how we can represent them. All I can say is, these matters have been well thought out, and you'd profit by reading as much as you can on the subject and by trying to avoid getting too caught up in your preconceptions.

I could almost convince myself that you know something I don't about the way calculators work, but after the 12-year-old comment by "best experts" was never backed up by anything, I had to jump ship. Where are you pulling this stuff?

This old post led me to an interesting question: will AI find itself in the position of our fictional philosophers of addition? The basic four functions of arithmetic are so fundamental to the operation of the digital computer that an intelligence built on digital circuitry might well have no idea of how it adds numbers together (unless told by a computer scientist, of course).

Bog: You are correct. That is, you do not understand this article at all. Pay attention to the first word, "Suppose..."

We are not talking about how calculators are designed in reality. We are discussing how they are designed in a hypothetical world where the mechanism of arithmetic is not well-understood.

"Like shooting blindfolded at a distant target"

So long as you know where the target is within five feet, it doesn't matter how small it is, how far away it is, whether or not you're blindfolded, or whether or not you even know how to use a bow. You'll hit it on a natural twenty.

4Eliezer Yudkowsky11yLogical fallacy of generalization from fictional evidence.
2MichaelHoward11yDamn right. And the same goes for the oft-quoted "million-to-one chances crop up nine times out of ten".

Thread necromancy:

Suppose that human beings had absolutely no idea how they performed arithmetic. Imagine that human beings had evolved, rather than having learned, the ability to count sheep and add sheep. People using this built-in ability have no idea how it worked, the way Aristotle had no idea how his visual cortex supported his ability to see things. Peano Arithmetic as we know it has not been invented.

It occured to me that a real life example of this kind of thing is grammar. I don't know what the grammatical rules are for which of the words "I" or "me" should be used when I refer to myself, but I can still use those words with perfect grammar in everyday life*. This may be a better example to use since it's one that everyone can relate to.

*I do use a rule for working out whether I should say "Sarah and I" or "Sarah and me", but that rule is just "use whichever one you would use if you were just talking about youself". Thinking about it now I can guess at the "I/me" rule, but there's plenty of other grammar I have no idea about.

-2Mass_Driver10yCan we get a link to the original thread?
6JoshuaZ10yIt this thread itself. He's commenting on the top paragraph of the original post. (It seems like thread necromancy at LW is actually very common. It may not be a good term given the negative connotations of necromancy for many people. Maybe thread cryonic revival?)
3DanielLC10yI'd expect here we'd give necromancy positive connotations. Most of the people here seem to be against death. I thought it's only thread necromancy if it moves it to the front page. This website doesn't seem to work like that. I hope it doesn't work like that, because I posted most of my comments on old threads.
2JoshuaZ10yJust because we have a specific attitude about things doesn't mean we need to go and use terminology that has pre-existing connotations. I don't think for example that calling cryonics "technological necromancy" or "supercold lichdom" would be helpful to getting people listen although both would be awesome names. However, Eliezer seems to disagree at least in regards to cryonics in certain narrow contexts. See his standard line when people ask about his cryonic medallion that it is a mark of his membership in the "Cult of the Severed Head." There's actually a general trend in modern fantasy literature to see necromancy as less intrinsically evil. The most prominent example would be Garth Nix's "Abhorsen" trilogy and the next most prominent would be Gail Martin's "Chronicles of the Necromancer" series. Both have necromancers as the main protagonists. However, in this context, most of the cached thoughts about death still seem to be present. In both series, the good necromancers use their powers primarily to stop evil undead and help usher people in to accepting death and the afterlife. Someone should at some point write a fantasy novel in which there's a good necromancer who brings people back as undead. Posts only get put to the main page if Eliezer decides do so (which he generally does to most high ranking posts).
2taryneast10yI dunno - I reckon you might get increased interest from the SF/F crowd. :)
1lessdazed9y...or would they [] ...nahh [].
1Luke_A_Somers9yFunny. I was working on something an awful lot like that back in 2000. I wasn't terribly good at writing back then, unfortunately.
1Oscar_Cunningham10yThere should be one on whatever page you're viewing my comment in (unless you're doing something unusual like reading this in an rss reader) Still, here you go: link []

McDermott's old article, "Artificial Intelligence and Natural Stupidity" is a good reference for suggestively-named tokens and algorithms.

Someone needs to teach them how to count: {}, {{}}, {{},{{}}}, {{},{{}},{{},{{}}}}...

5[anonymous]9yeven less esoteric: |, ||, |||, ||||, |||||, .... Then "X" + "Y" = "XY". For example |||| + ||| = |||||||. It turns out the difficulty in addition is the insight that ordinals are just an unfriendly representation. One needs a map between representations in order that the addition problem becomes trivial.

when you need to publish at least one paper per month

Gah! Any field with a publishing requirement like that... I shudder.

And... is it me, or is this one of the stupidest discussion threads on this site?

"I don't think we have to wait to scan a whole brain.  Neural networks are just like the human brain, and you can train them to do things without knowing how they do them.  We'll create programs that will do arithmetic without we, our creators, ever understanding how they do arithmetic."

This sort of anti-predicts the deep learning boom, but only sort of.

Fully connected networks didn't scale effectively; researchers had to find (mostly principled, but some ad-hoc) network structures that were capable of more efficiently learning complex patterns.

A... (read more)