2,657 thoughts on “Elon Musk Thinks Evolution is Bullshit.”

  1. walto:
    keiths,

    KN is right, of course.You’ve presented NO arguments for anything. You’ve simply repeated claims that are contrary to what nearly everybody means by “know.”Claims according to which nobody knows his or her own name. He’s right too, that you generally don’t reply to questions, but simply attack those who don’t agree with you.

    In a word, as those who have dealt with you will all know, KN’s post was right on target.And he is among the most gentle posters one can find on this site.

    Yes, indeed. I enjoy and appreciate KN’s contributions here, even though I’m often remiss in thinking but not saying it. There were a couple of points he made earlier I intend to pick up on when I have more time.

  2. walto: Sometimes we think we know stuff when we don’t. That’s true. But sometimes we think we know stuff and we’re right. Can we always tell the difference between those states? No.

    On your view, are there cases where we do have knowledge but don’t realize it?

  3. Kantian Naturalist: On your view, are there cases where we do have knowledge but don’t realize it?

    Sure. I don’t think one even has to have the concept of knowledge to know things.

  4. walto: Sure.I don’t think one even has to have the concept of knowledge to know things.

    Right! Animals and humans knew lots of stuff long before the word “know” was coined in any language.

  5. Exactly. (Although I don’t really have a sense about what animals know. Different forms of life and all that…..)

  6. keiths:

    The chess game is part of the universe. At some point in time the state of the universe includes a chess board with a half-dozen pieces remaining in an unfinished game. Someone steps into the room to find both players have suffered fatal aneurisms simultaneously. There were no other observers of the game. There is no way to tell from the current state of the universe how the pieces came to be in their current configuration.

    Keep in mind the difference between state and quantum state. From your perspective within the universe, some wavefunctions have collapsed, and it is wavefunction collapse that introduces indeterminacy. You can’t deduce the history from a collapsed wavefunction, even in principle.

    It is the universe’s uncollapsed wavefunction that preserves its history and determines its future.

    Sprinkling quantum woo on an argument does not make it magically compelling. I’ve provided several examples of how different histories can arrive at the same current state. I see no evidence for the preservation of those distinct histories in quantum mechanics. It does not appear to be possible, even in principle, to deduce a unique history from a current state.

    If you have actual evidence or a detailed argument based on known physics supporting the alternative position, I’d be very interested in seeing it.

  7. BruceS:
    While I am on the topic of confusions, some of P’s posts seem to me to confuse (practical) predictability and determinism. They are not the same thing. Determinism assumes perfect knowledge of the relevant type of initial state (which for QM is a quantum state).

    Laws of physics at the basic level, both quantum and classical, are time reversible, as long as one understands the nature of the state being described by those laws*.

    I am trying to address the claim that a unique history can be determined, in principle, from a current state. Thus far I haven’t seen an argument to support this that addresses my examples of multiple histories leading to the same state. I don’t believe that quantum mechanics resolves this problem, but I’m willing to be convinced if you have mathematical or physical proof.

  8. Patrick: I am trying to address the claim that a unique history can be determined, in principle, from a current state.Thus far I haven’t seen an argument to support this that addresses my examples of multiple histories leading to the same state.I don’t believe that quantum mechanics resolves this problem, but I’m willing to be convinced if you have mathematical or physical proof.

    You cannot have many states leading to one if laws are time reversible and deterministic in time. For if two states in the past led to one state now, then going backwards in time would create an one state leading to two, which is not allowed. Time reversible means that going backwards and forwards must be indistinguishable.

    For QM in particular, state means quantum state (not the result of measuring a state). Determinism both ways in time is a property of the math of QM, specifically, the linearity of the formalism and the requirement that operators be unitary. I am talking about QM as presented in introductory versions, not the quantum field version. Although there too QM is time reversible (except for the weak force, which is CPT-reversible), I cannot provide the reasoning why that is.

    This is all common knowledge about QM AFAIK. For an introductory proof outline of the non-QFT formulation of QM (which is my level of knowledge). see Susskind’s book on QM, pages 93-98. From page 97:

    : … the minus first law underlies everything else. It says information is never lost. If two identical systems start out in different states, they stay in different states. Moreover, in the past they were also in different states. … Distinctions are preserved. The quantum version of the minus first law has a name — unitarity.

    Specifically, unitarity ensures that two orthogonal vectors stay orthogonal in both directions of time. Since quantum states are vectors which can always be written in terms of an orthonormal basis, that with unitarity of operators gets the result I claim.

    ETA: Susskind’s lecture 1 about classical mechanics on YT mentions this minus first law as well, as I recall.

    ETA: I have not looked at your examples, but I agree that one can create systems of dynamic behavior where what you claim happens. Susskind does so in the lecture on YT. But the laws of physics for classical mechanics and QM have constraints (on their mathematical form) which prevent this type of situation.

  9. BruceS,

    Bruce, do you not see any tension between your swampy examples and a reversibility/unitarity that seems to imply that one can’t have qualitative identity with different causal backgrounds?

    (AKA what am I missing here??)

  10. BruceS: You cannot have many states leading to one if laws are time reversible and deterministic in time. For if two states in the past led to one state now, then going backwards in time would create an one state leading to two, which is not allowed.Time reversible means that going backwards and forwards must be indistinguishable.

    I’m not sure that your logic is sound there, but I need to think on it.

    For QM in particular, state means quantum state (not the result of measuring a state).Determinism both ways in time is a property of the math of QM, specifically, the linearity of the formalism and the requirement that operators be unitary. I am talking about QM as presented in introductory versions, not the quantum field version.Although there too QM is time reversible (except for the weak force, which is CPT-reversible),I cannot provide the reasoning why that is.

    It’s been a few years since I was last in a QM class, but my understanding is that, while the math allows that operation, it hasn’t been demonstrated to reflect physical reality. Am I incorrect?

  11. Patrick: It’s been a few years since I was last in a QM class, but my understanding is that, while the math allows that operation, it hasn’t been demonstrated to reflect physical reality. Am I incorrect?

    Excuse me for butting in, but I’d say so too (ie that you are correct). I already noticed Bruce’s remark that

    …the laws of physics for classical mechanics and QM have constraints (on their mathematical form) which prevent this type of situation.

    which seems to overlook the fact that scientific laws are descriptions or models of reality. Another map/territory issue.
    *looks sidelong at walto*

  12. walto:
    Bruce, do you not see any tension between your swampy examples and a reversibility/unitarity that seems to imply that one can’t have qualitative identity with different causal backgrounds?

    As I understand the concern, we are saying that SM matches me at molecular even quantum level but possesses a different history at quantum level than me. Further, the causal history of me expressed in chemistry, biology, psychology, ethology supports the norms needed to say I understand a language but the history of SM does not.

    The seems to contract the claim that if two entities are at the same quantum state, they must be shared all states in the past. So how can causal histories which supervene on those past states differ?

    Is that your concern?

    If so, I would say that the we need to consider the state vector relevant to that whole historical time which would include more of the environment that just my body versus SM’s. It would support the different causal histories at higher levels of scientific description. The stipulation that SM was created by a quantum fluctuation whereas I know I was not supports that view (ETA:) that the relevant state vector requires a bigger system than just my body versus SMs on which to based the different histories.

  13. Patrick: I see no evidence for the preservation of those distinct histories in quantum mechanics. It does not appear to be possible, even in principle, to deduce a unique history from a current state.

    At the risk of mangling my quantum woo, I offer up the non-deleting theorem.

    “in general, given two copies of some arbitrary quantum state, it is impossible to delete one of the copies.”

  14. Alan Fox: Excuse me for butting in, but I’d say so too (ie that you are correct). I already noticed Bruce’s remark that

    which seems to overlook the fact that scientific laws are descriptions or models of reality. Another map/territory issue.
    *looks sidelong at walto*

    Do you have a better description of how reality behaves? Nothing better than QM exists for prediction and retrodiction.

    The constraints I mention have worked so well in creating the success of science that I think you need a very good reason indeed to deny them. So then it comes down to what the incredible success of science allows us to say about the nature of reality. I did already mention this issue in my earlier posts.

    There is no reason whatsoever to think that reality behaves in a way consistent with dynamic systems of the sort Patrick envisions. And every reason to believe it does not, assuming one accepts science as the only way to understand how reality behaves.*

    ETA: sure the map is different from the reality. That does not mean the map does not tell us how reality is in a way that is relevant to the issue at hand.

    * ETA: On re-reading this, I see it sounds scientistic. That was not my intention. I was referring to science as the best way to predict, control, and explain those aspects of reality relevant to claims of determinism.

  15. Patrick: I’m not sure that your logic is sound there, but I need to think on it.

    It’s been a few years since I was last in a QM class, but my understanding is that, while the math allows that operation, it hasn’t been demonstrated to reflect physical reality.Am I incorrect?

    Draw a state diagram for the dynamics of a system which has arrows from two states leading to the same subsequent state. Reversing time reverses those arrows, which means there is unacceptable ambiguity in the time evolution (with time running in reverse direction). Susskind does that exercise in the video.

    On “affecting physical reality”: see my post to Alan just above.

  16. walto: I agree with you that petrushka has been confounding predictability and determinism (or map with territory) in a couple of his posts. A bunch of Alan’s remarks on this thread have done that too, I think. It’s one of the most common mistakes people make when they discuss philosophy.

    I admit to not giving the thread as much attention as it deserves. But I have been giving it more attention than I can really afford. I’m surprised I’ve given the impression of confusing logic and math with reality, though. That logic or math can trump evidence-based investigation of reality is a pet peeve.

    Alan’s whole “light cone” confab required that confusion–which I think keiths pointed out quite well.

    I think the light cone is a neat illustration of the practical limits to our ability to find out about ourselves and our environment. If one takes as a premise (or assumes or defines) that the universe is “everything that exists” sure we can draw a sound logical conclusion depending on that premise. But I’m more interested in what reality is. The reality is the velocity of light is finite. We don’t know what is beyond the limit of the past light cone. It could be an infinite amount of the same or something completely different.

  17. BruceS: Do you have a better description of how reality behaves? Nothing better than QM exists for prediction and retrodiction.

    I’m sure it is the best predictive model we have. I certainly don’t have a better one.

    The constraints I mention have worked so well in creating the success of science that I think you need a very good reason indeed to deny them. So then it comes down to what the incredible success of science allows us to say about the nature of reality. I did already mention this issue in my earlier posts.

    No argument from me on what you say here. I absolutely agree.

    There is no reason whatsoever to think that reality behaves in a way consistent with dynamic systems of the sort Patrick envisions.

    Patrick, if I understood him correctly, was pointing out that the past cannot be known precisely from information, however detailed, from the present. And that seems correct to me. Travel into the future is not paradoxical. Time travel into the past on the other hand…

    And every reason to believe it does not, assuming one accepts science as the only way to understand how reality behaves.

    I’m not sure where our disagreement lies, now!

  18. Alan Fox: . Travel into the future is not paradoxical. Time travel into the past on the other hand…

    I do agree time travel into the past and future is a different issue than retrodiction being just as reliable as prediction according to our best science.

  20. walto,

    KN is right, of course. You’ve presented NO arguments for anything.

    I’ll let you fight it out with KN, who recognizes my argument and has been trying to counter it. (Or was, until he flounced.)

    And he is among the most gentle posters one can find on this site.

    He’s gentle when he’s being gentle, and he’s prickly and aggressive when he’s being prickly and aggressive.

    Remember, it was KN who pulled this stunt:

    I deleted all the OPs I’d started in a fit of rage, and now that I’ve had a long walk and calmed myself down, I don’t care that they’ve been restored or not.

    Deleting all your OPs (and the comments!) in a fit of rage is not gentle behavior.

    Note that even after he had calmed down, he was still saying “I don’t care whether they’ve been restored” to the rest of us whom he had shat upon. It was all about him.

    If that’s gentleness, then we need less gentleness at TSZ.

    It’s interesting, and quite convenient, that KN decided to flounce just as I was pressing for an answer to my question. Since you’re defending him, perhaps you can supply the answer that he cannot:

    Why resort to a invalid circular argument against Cartesian skepticism when a valid non-circular argument for it is available?

  21. Alan,

    Patrick, if I understood him correctly, was pointing out that the past cannot be known precisely from information, however detailed, from the present. And that seems correct to me. Travel into the future is not paradoxical. Time travel into the past on the other hand…

    Surely you recognize that predicting the future is not the same as traveling to it, no? Ditto for recovering the past.

  22. Alan Fox: That logic or math can trump evidence-based investigation of reality is a pet peeve.

    Alan’s whole “light cone” confab required that confusion–which I think keiths pointed out quite well.

    I think the light cone is a neat illustration of the practical limits to our ability to find out about ourselves and our environment. If one takes as a premise (or assumes or defines) that the universe is “everything that exists” sure we can draw a sound logical conclusion depending on that premise. But I’m more interested in what reality is.

    It’s not a matter of “trumping.” When we say, “Suppose the universe is F,” we’re not saying that it’s possible we might ever know that the universe is F or that we can tell if the universe is F or that we can ever find out that the universe is F. You can think of it as pretending if you like. But it’s important not only for philosophy, but for science that we can do this.

    When you say you’re more interested in what reality is, that’s fine. But it’s important not to confuse what we do or can know with what is–whether we can ever know anything about it or not. As keiths suggested, knowing that all bachelors are unmarried does not involve knowing a single contingent fact. You don’t have to know that there are any bachelors or even if there has ever been one. There’s a sense in which knowing it isn’t knowing a single thing about the world. But it’s true nevertheless.

    We can talk about the universe outside the light cone even if we can never experience it: that’s one of the interesting things about the human mind. And, in fact, that’s what cosmologists like Hawking do all the time.

  23. keiths:
    walto,

    I’ll let you fight it out with KN, who recognizes my argument and has been trying to counter it.(Or was, until he flounced.)

    He’s gentle when he’s being gentle, and he’s prickly and aggressive when he’s being prickly and aggressive.

    Remember, it was KN who pulled this stunt:

    Deleting all your OPs (and the comments!) in a fit of rage is not gentle behavior.

    Note that even after he had calmed down, he was still saying “I don’t care whether they’ve been restored” to the rest of us whom he had shat upon.It was all about him.

    If that’s gentleness, then we need less gentleness at TSZ.

    It’s interesting, and quite convenient, that KN decided to flounce just as I was pressing for an answer to my question.Since you’re defending him, perhaps you can supply the answer that he cannot:

    Why resort to a invalid circular argument against Cartesian skepticism when a valid non-circular argument for it is available?

    I’ll respond to this when you either admit that you actually know your name or admit that you don’t believe that you do know your own name.

  24. BruceS: Is that your concern?

    Yes, that was it. I’ll have to think about your response. Thanks.

    ETA: I now think that if you could flesh that out with a particular SM example, it might be helpful.

    I mean, I’m either identical to my SM or I’m not. Unitarity requires that I’m not. We know that on your view there are differences in linguistic capabilities, as a result of different causal histories. So now we need an argument that the difference(s) in our linguistic capabilities cannot be a function of our (now conceded) internal differences, or externalism is arguably not necessary to explain those alleged differences. (My initial sense is that the moral is that SM examples ought to be dropped if unitarity is accepted: they just confuse matters.)

  25. BruceS: …retrodiction being just as reliable as prediction according to our best science.

    Perhaps I should mention I don’t disagree with this either/ Postdiction is a powerful tool. Neil Shubin’s search for Tiktaalik is a shining example that comes to mind.

  26. Bruce:

    You cannot have many states leading to one if laws are time reversible and deterministic in time. For if two states in the past led to one state now, then going backwards in time would create an one state leading to two, which is not allowed.Time reversible means that going backwards and forwards must be indistinguishable.

    Patrick:

    I’m not sure that your logic is sound there, but I need to think on it.

    Bruce is right, Patrick. Here’s a software analogy:

    I assign a value to a 32-bit variable:

    refresh_interval = 30000;

    Before that statement executes, ‘refresh_interval’ might be in any one of 2^32 states. After the statement executes, it is in exactly one: it contains the value 30000. There are 2^32 possible previous states, all funneling into the same current state.

    If you try to mentally run that process backwards, you can’t do it. Having overwritten the variable ‘refresh_interval’, you have lost the information about what it previously contained. Without that information, you can’t go backwards.

    The “laws” of variable assignment are not time-reversible because there is not a unique previous state for every current state.

  27. walto,

    I’ll respond to this when you either admit that you actually know your name or admit that you don’t believe that you do know your own name.

    What’s to admit? I do know my name and have never claimed otherwise. I’m not absolutely certain of it, of course — absolute certainty is mythical.

    Now, can you answer the question that KN cannot?

  28. keiths,

    Wait. How can you know your own name, given that you might be deceived by a demon or be a brain in a vat?

    What’s the difference between knowing your name and knowing that you’re seeing a tree?

  29. walto: It’s not a matter of “trumping.” When we say, “Suppose the universe is F,” we’re not saying that it’s possible we might ever know that the universe is F or that we can tell if the universe is F or that we can ever find out that the universe is F. You can think of it as pretending if you like. But it’s important not only for philosophy, but for science that we can do this.

    I’m not the thought police. Without checking, I think the thrust of my initial comments to Keiths’ claim that thought experiments were “important” was “what are the consequences?” That hasn’t been answered satisfactorily so far, as far as I am aware.

    When you say you’re more interested in what reality is, that’s fine. But it’s important not to confuse what we do or can know with what is–whether we can ever know anything about it or not. As keiths suggested, knowing that all bachelors are unmarried does not involve knowing a single contingent fact. You don’t have to know that there are any bachelors or even if there has ever been one. There’s a sense in which knowing it isn’t knowing a single thing about the world. But it’s true nevertheless.

    I don’t think I’m confused on that. There is an equivocation on truth (maybe I’m missing a point, though). Regarding categories by definition, sure. But most real entities have fuzzy edges.

    We can talk about the universe outside the light cone even if we can never experience it: that’s one of the interesting things about the human mind. And, in fact, that’s what cosmologists like Hawking do all the time.

    Well, of course. We can construct hypotheses, except there is no way to test them.

  30. keiths: Surely you recognize that predicting the future is not the same as traveling to it, no? Ditto for recovering the past.

    Surely I’ve given you no reason to think so? Why ask if not to provoke?

  32. BruceS:

    It’s been a few years since I was last in a QM class, but my understanding is that, while the math allows that operation, it hasn’t been demonstrated to reflect physical reality.Am I incorrect?

    On “affecting physical reality”:see my post to Alan just above.

    Not “affecting”, “reflecting”. Again, my understanding is that while the math allows for a time reversal operator, this does not reflect what we observe in the real world. QM is certainly highly predictive in many areas, but this seems to be an instance where it is not.

    As I said, it’s been a few years since I last cracked a QM textbook, so my information could be out of date.

  33. keiths:
    Bruce is right, Patrick. Here’s a software analogy:

    I assign a value to a 32-bit variable:

    Before that statement executes, ‘refresh_interval’ might be in any one of 2^32 states.After the statement executes, it is in exactly one:it contains the value 30000.There are 2^32 possible previous states, all funneling into the same current state.

    If you try to mentally run that process backwards, you can’t do it.Having overwritten the variable ‘refresh_interval’, you have lost the information about what it previously contained.Without that information, you can’t go backwards.

    The “laws” of variable assignment are not time-reversible because there is not a unique previous state for every current state.

    That’s not where I think the problem may lie. Bruce and you, if I understand you correctly, are saying that every current state of the universe has a unique history that can be determined, in principle if not practice, from that state alone. I am unconvinced that is true, even taking QM into account. It’s certainly not true in the practical cases I mentioned. I need to ponder it more.

  34. walto: Y

    I mean, I’m either identical to my SM or I’m not.Unitarity requires that I’m not.

    I don’t agree with that.

    I and SM are not closed systems, so in considering our history, any outside system which could influence that history, eg entanglement for QM, must be considered.

    So that would bring us to the fun issue of how classical reality emerges for a subsystem of a larger quantum system. I’ll go with decoherence and call it a day.

  35. BruceS: I mean, I’m either identical to my SM or I’m not.Unitarity requires that I’m not.

    I don’t agree with that.

    I and SM are not closed systems, so in considering our history, any outside system which could influence that history, eg entanglement for QM, must be considered.

    Can you flesh that out a bit more? How can you and your swamp double be (qualitatively) identical given different histories and unitarity?

    Thanks.

  36. Patrick:
    That’s not where I think the problem may lie. Bruce and you, if I understand you correctly, are saying that every current state of the universe has a unique history that can be determined, in principle if not practice, from that state alone

    Hey, its not me, it’s Lenny Susskind and just about every physicist out there. I am taking their word for it; I have only a very basic understanding of the math and physics underlying the justification and explanation.

    One does have to define state properly and also accounts for outside factors if one is looking at an open subsystem (and its pretty hard to get true closed systems in QM, which is Scott A has pointed out, is one reason why quantum computers are such a challenge).

    I’m not sure if it helps, but as I noted previously, conservation of energy is equivalent to time reversibility (here for caveats and details). Conservation of energy is usually considered a good thing to support in one’s theories.

  37. walto:
    Alan Fox,

    I make no claim that anything I’ve ever written or said is “important.”Mostly I go for pithy.😉

    I’m usually going for irony. 😉

  38. BruceS: …time reversibility…

    Thé impossibility of time travel into thé past is where I suggest models that are time-réversible are not accurate models of reality.

  40. walto: Can you flesh that out a bit more?How can you and your swamp double be (qualitatively) identical given different histories and unitarity?

    Thanks.

    Time reversibility only works for closed systems.

    There is nothing preventing a subset of a closed system achieving the same state as a subset of a different closed system, even though the history of the largest enclosing closed systems for each may differ.

    For QM, entanglement makes identifying the largest enclosing closed system challenging. Possibly one needs the whole universe? I don’t know.

  41. Alan Fox: Thé impossibility of time travel into thé past is where I suggest models that are time-réversible are not accurate models of reality.

    I’m just using the model for retrodiction and in particular for stating that different current states necessitate different past states. The fact that I could not time travel there to check is not relevant.

  42. BruceS: Time reversibility only works for closed systems.

    There is nothing preventing a subset of a closed system achieving the same state as a subset of a different closed system, even though the history of the largest enclosing closed systems for each may differ.

    For QM, entanglement makes identifying the largest enclosing closed system challenging.Possibly one needs the whole universe?I don’t know.

    Ok, thanks. That’s not quite leibnizian monad land then, so I think it’s consistent with patrick’s examples of failures of reversibility. The similar results can have different pasts because they aren’t ‘closed systems.’

    (I’m taking on faith that ‘closed’ can be defined without reference to unitarity.)

  43. Minor aside: there are actual two (at least) theories of fundamental physics: quantum mechanics and general relativity. General relativity is (so far as I know) fully deterministic. There are both deterministic and non-determinstic interpretations of QM that are empirically equivalent (given current evidence and theory). Our current understanding of physical reality does not permit us to reconcile general relativity and quantum mechanics. Under these epistemic constraints, any argument for or against determinism is going to face significant obstacles.

    Of course one can do metaphysics without worrying at all about the correlated and complementary epistemology, but quite frankly I don’t see that approach as methodologically different from theology, even if it is substantively atheistic or naturalistic.

  44. KN,

    The reliability of perception isn’t an assumption; it’s supported by embodied cognitive science. (Though arguably embodied cognitive science lightly revises the meaning of “the reliability of perception”.)

    Of course that point won’t convince the foundationalist who points out that science depends on the reliability of the senses. But to the anti-foundationalist — whether an epistemic holist like Sellars and Michael Williams or a ‘foundherentist’ like Haack — there’s nothing amiss with provisionally using our senses to explore the world around us, using our growing knowledge of the world to explore how our brains and senses work, and thereby both explaining why our senses are reliable (to the extent that they are) and perhaps also revising the meaning of “the reliability of the senses.”

    Science depends not just on the reliability of the senses (in the watered-down way in which you’re using the word), but also on their veridicality. I made that point in an earlier exchange:

    KN:

    By contrast, “reliable” invites the question, “reliable for what?” — to which one natural response is, “reliable for guiding actions that contribute to the satisfaction of the organism’s goals”.

    keiths:

    You’re undermining your position. That’s actually an argument for using “veridical” instead of “reliable”! Cartesian skepticism is an epistemological stance, after all, so we are concerned about what is true, not what is useful.

  46. BruceS: I’m just using the model for retrodiction and in particular for stating that different current states necessitate different past states.

    Again I’d agree with you to the extent that in reality we can’t have mathematical identity so any thought experiment such as “swamp-man” or “twin Earth” is utterly pointless in my view. What I query is that pasts are “veridically” engraved on entities in the present. One may claim there was one and only one single path of history for that present object but I suggest it is not possible to accurately know that history just from its present existence.

  47. Alan:

    Patrick, if I understood him correctly, was pointing out that the past cannot be known precisely from information, however detailed, from the present. And that seems correct to me. Travel into the future is not paradoxical. Time travel into the past on the other hand…

    keiths:

    Surely you recognize that predicting the future is not the same as traveling to it, no? Ditto for recovering the past.

    Alan:

    Surely I’ve given you no reason to think so?

    Surely you have. This quote provides a very good reason to think so:

    Patrick, if I understood him correctly, was pointing out that the past cannot be known precisely from information, however detailed, from the present. And that seems correct to me. Travel into the future is not paradoxical. Time travel into the past on the other hand…

    Anyway, I think I’ve identified the source of your confusion. When the rest of us talk about the time-reversibility of the laws of physics, you actually think we’re saying that time can flow backwards. That’s not it at all!

    This comment of yours confirms my diagnosis:

    Thé impossibility of time travel into thé past is where I suggest models that are time-réversible are not accurate models of reality.

    Time reversibility just means that if you change the sign of the time parameter in all of the appropriate variables, then the laws remain true.

  48. Bruce:

    You cannot have many states leading to one if laws are time reversible and deterministic in time. For if two states in the past led to one state now, then going backwards in time would create an one state leading to two, which is not allowed.Time reversible means that going backwards and forwards must be indistinguishable.

    Patrick:

    I’m not sure that your logic is sound there, but I need to think on it.

    keiths:

    Bruce is right, Patrick. Here’s a software analogy:

    I assign a value to a 32-bit variable:

    refresh_interval = 30000;

    Before that statement executes, ‘refresh_interval’ might be in any one of 2^32 states. After the statement executes, it is in exactly one: it contains the value 30000. There are 2^32 possible previous states, all funneling into the same current state.

    If you try to mentally run that process backwards, you can’t do it. Having overwritten the variable ‘refresh_interval’, you have lost the information about what it previously contained. Without that information, you can’t go backwards.

    The “laws” of variable assignment are not time-reversible because there is not a unique previous state for every current state.

    Patrick:

    That’s not where I think the problem may lie.

    You said you were unsure of the soundness of Bruce’s logic, so I gave an example demonstrating it.

  49. walto,

    How can you know your own name, given that you might be deceived by a demon or be a brain in a vat?

    What’s the difference between knowing your name and knowing that you’re seeing a tree?

    The latter depends on the veridicality of the senses, but the former doesn’t.

    I really wish you’d read my comments. As I keep telling you, I’ve addressed the intellect vs senses issue already:

    That might well be true. I already rule out absolute certainty, even of “self-evident” truths, on those grounds. The only reason I don’t rule out knowledge of them, as well, is that the apparent consistency of our reasoning makes it likely to be largely correct, in my estimation. (Consistency has different implications for reasoning than it does for the senses. I can elaborate on that if needed.)

    I could be wrong about that, though, and maybe even knowledge of something as basic as “a thing is identical to itself” is impossible. If so, no crisis. I already attach an implicit “if my senses are veridical” to the word “know” in cases that require it, so I can easily do the same with “if my reasoning is correct”.

  50. walto,

    With that out of the way, how about tackling the question that KN couldn’t answer, as you said you would?

    Why resort to a invalid circular argument against Cartesian skepticism when a valid non-circular argument for it is available?

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