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I am reading "How Physics Makes Us Free" and have a question about the central Daniel Dennett thought experiment in the opening chapter. The experiment treats body parts, crucially the brain, as a component of the body like a spark plug in a car (brain in a vat). It is, rather, part of an organism and in my mind indivisible from the nervous system. Even when higher brain function is dead a body will still reject a donated organ and attack it as alien. A thousand same-model spark plugs will work in a car without any issues. It is at the level of biology that identity first appears. Yet the thought experiment treats physics and psychology as the only relevant domains. If the thought experiment were true to biology it would not be enough to replicate all the synapses and nerves but the entire body as the biological instantiation of identity. Am I overstating a life-science claim to some part of this scenario?

You give an interesting argument that the ground of one's identity is biological rather than (just) physical and/or psychological. But it may run into a problem. Not only can one's body reject organs transplanted from someone else. It can also, in the case of autoimmune disease, "reject" (i.e., attack) one's own cells and tissues: sometimes the body doesn't "know its own." Yet it seems incorrect to say that sufferers of autoimmune disease have a "compromised" identity. Does this problem cast doubt on your proposal?

Some people define some things (which they truly may be or are) Impossible. 'Impossible' has a humane meaning in itself. But... If 'something' is really impossible... then why can you think that? If something is impossible... then why did the neurons in your brain have that thought? It must've been impossible for them to think of something which is not possible.

I'll assume, just for simplicity, that by a "thought" you mean a belief and by "something impossible" you mean a proposition that cannot possibly be true . I hope my assumptions aren't off the mark. (I'm not a neuroscientist, so I'll say nothing about how neurons work.) If my assumptions are correct, then your question becomes "How can anyone believe a proposition that cannot possibly be true?" One answer is this: "Easy! For example, many people down through the ages believed that they had accomplished the famous geometric construction known as squaring the circle . But the proposition they believed cannot possibly be true, because squaring the circle is impossible, as was finally proven in 1882. Those who believed the proposition obviously didn't see the impossibility of the construction." An opposing answer is this: "They can't! Indeed, we can understand the behavior of those misguided geometers only if we attribute to them a false belief that could have been true, such as the belief that a...

We know for now, at least, it's impossible to go back in time scientifically. But what if you really needed to, say if you had done something really bad and had ever desperation to go back in time and correct what you did, so you don't suffer the consequences you are suffering in the present. Provided you would not cause a disaster by going back in time, and that you would only change the bad things you did, it is an interesting concept. With this context, if you could be given a drug, that would leave you asleep for the rest of your life (coma), would you do it? Read on, there's more. In this sleep, you will have a dream, which is set from just before your mistake. So essentially, it causes you to simulate the past and the rest of your life in your head. It seems real, but it isn't. My question is, would this be the same as going back in time and changing things in reality? Does reality matter more, or our interpretation of it?

First a terminological quibble. By "scientifically impossible," I take it you really mean just "technologically infeasible," i.e., impossible given the limits of current technology. As I see it, what's scientifically possible or impossible depends only on the laws of nature, which are standardly regarded as unchanging over time (or at least over any time that humans will experience). I think the jury's still out on whether backward time-travel is scientifically impossible in this latter sense. To your question: I think there's something self-contradictory in the idea of "correcting what you did" if that means "bringing it about that you never did what you in fact did." Either (1) you did it, or (~ 1) you never did it. I can't see how any consistent story features both (1) and (~ 1). In that sense, then, there's no such thing as (2) "going back in time and changing things in reality" and therefore nothing that's "the same as" (2). See section 1.2 of the SEP article on time-travel .

Do most philosophers take the Paul and Patricia Churchland's eliminative materialism seriously? I'm concerned about the current state of philosophy of mind in that it seems that at least some people take seriously the suggestion that e.g. beliefs don't exist (and that they are believed in in a theoretical manner). So, again, how popular is the Churchland's eliminative materialism in contemporary philosophy of mind?

I'm not sure why you regard it as a worrisome sign about current philosophy of mind that some of its practitioners take eliminative materialism (EM) seriously. At worst, it would show that some philosophers regard EM as far more plausible than it really is, but even then I don't see how that would indict current philosophy of mind as a whole. Anyway, you've asked an empirical question whose answer depends on (1) reliable data about the views of philosophers and (2) what you mean by "take seriously." I presume you mean something like "regard as too plausible to be dismissed without argument," which is a weaker attitude than "regard as plausible." I don't have empirical data, but my hunch is that most philosophers do take EM seriously in that sense, but probably because most philosophers don't regard any philosophical position as worthy of being dismissed literally without argument. At the same time, however, my hunch is that most philosophers regard EM as implausible. Curiously, the PhilPapers...

There has been much made of Hawking and Harris using brain scans to demonstrate a deterministic explanation of "free will". My question is, how do they treat a case where I think about moving my arm, but don't? How can the experiment they site test thoughts, subjective experience, etc. which do not lead to any outward physical effects? Must we accept that for all cases of mental phenomena that the brain scan test constitutes a proof?

Must we accept that for all cases of mental phenomena ... the brain scan test constitutes a proof? I think the important philosophical question here is "A proof of what?" Suppose that science did somehow establish that all of our choices are causally determined by our earlier brain states. According to compatibilism , that result wouldn't threaten free will at all, and according to many compatibilists it would be good news indeed for free will. Before we get too hung up on whether brain scans are evidence in favor of determinism about human choices, let's ask the prior philosophical question "What difference would that make for free will?" You'll find this issue discussed many times on this website, including here: Question 5451 Question 5711

Is mathematics independent of human consciousness?

I'm strongly inclined to say yes . Here's an argument. If there's even one technological civilization elsewhere in our unimaginably vast universe, then that civilization must have discovered enough math to produce technology. But we have no reason at all to think that it's a human civilization, given the very different conditions in which it evolved: if it exists, it belongs to a different species from ours. So: If math depends on human consciousness, then we're the only technological civilization in the universe, which seems very unlikely to me. Here's a second argument. Before human beings came on the scene, did the earth orbit the sun in an ellipse, with the sun at one focus? Surely it did. (Indeed, there's every reason to think that the earth traced an elliptical orbit before any life at all emerged on it.) But "orbiting in an ellipse with the sun at one focus" is a precise mathematical description of the earth's behavior, a description that held true long before consciousness emerged here....

The universe appears to behave in logical ways. All of the individual physical components of the universe, as far as we can tell, are likely governed by logically consistent laws of physics. According to physicalism, human beings are nothing more than complex physical systems. That means that the physical components and functions of a human being, including those that give rise to human thought, are governed by the same logically consistent laws that govern the behavior of electrons, etc. If the physical processes that give rise to thought are rational how can a human being have an irrational thought? Where in the system does irrationality arise? It seems that human beings are in fact capable of irrational thought. If two people hold mutually exclusive ideas then at least one of them must be wrong. But if irrational thought is possible where does it come from? Is this an argument against physicalism? Does it mean we are more than just bits of matter? Or does it mean that the universe itself doesn't...

You asked, "If the physical processes that give rise to thought are rational, how can a human being have an irrational thought?" You might be misinterpreting the claim that "the physical processes...are rational." Presumably what's meant by the claim is that the physical processes can be discovered and understood by rational means , such as empirical investigation and logical reasoning. The claim doesn't mean to attribute rationality to the physical processes themselves: the processes don't literally investigate or reason, either well or badly. So the fact that the physical processes can be discovered and understood rationally doesn't imply that irrational thoughts can't result from those processes. Furthermore, we can rationally investigate the physical causes of irrational thoughts, even if science isn't very far down that road at present. In any case, we should resist the suggestion that the universe sometimes violates the laws of logic: that suggestion is either impossible or not even...

Do you think that there are things humans cannot understand because our brains are limited? Philosopher Thomas Nagel was recently quoted as saying that there are surely truths that people cannot understand (and will never be able to understand), as "nine-year olds cannot understand Maxwell's equations". I don't think this is a good example: after all nine-year olds are very smart, and it seems to me that they just don't have the time and information to "understand Maxwell's equations" while they are still nine years old. Is there any reason why nine-year olds wouldn't understand those equations if they had a nine-year old brain (physically speaking) forever (always adding new information)? And what if such equations were explained to them? Anyway, I would like you to answer not about the example, but about the general issue. Of course there are things we will never know and cannot know (for instancel, many things that happened before humans existed, or in distant parts of the universe, or things people...

I'm inclined to think that there are, and perhaps must be, things that humans can't understand because of the limitations of our brains. Now, the term 'understand' might mean (i) 'understand at all (i.e., at least partly)' or (ii) 'understand completely'. On interpretation (ii), it's pretty clear that there are things we can't understand, because our finite brains can't grasp all of the infinitely many facts there are about even as ordinary a thing as my car. What's my car's exact mass right now? (No rounding allowed!) Even on interpretation (i), there may be things that humans can't understand. The only organ with which we can hope to understand something is our brain. Why wouldn't our brain's finite capacity for information storage, calculation, and so on, be accompanied by a finite capacity for understanding things? Indeed, Colin McGinn has conjectured that some of the central problems of philosophy have endured for so long because, although they have solutions, our species lacks the ability...

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