# How can we really know that time travel doesn't happen?

Well, for one piece of evidence, no confirmed time travelers attended the Time Traveler's Convention held at MIT in 2005 (http://web.mit.edu/adorai/timetraveler/ ). The organizers do note that some might have attended in disguise, to avoid questions about the future. More seriously, some philosophers have argued that time travel involves logical contradictions, and if that's right, then we can be sure that time travel doesn't happen. And certainly many time travel stories do seem to involve contradiction or incoherence. When Marty McFly travels back in time and then returns to the present day, the present day has been changed. So it looks as if 1985 the second time around differs from 1985 the first time around. But 1985 only occurs once, so it can only occur one way. You might think of this as "the second time around fallacy." Another famous contradiction discussed relative to time travel is the Grandfather Paradox. If time travel were possible, then you could go back in time and...

# We have no evidence whatsoever that the world will not wink out of existence tomorrow, or the day after that, or some other time in the future. Even if it doesn't, we accept that we may all die at any time, even if the chance of it is low for someone young and healthy who doesn't take many risks. Isn't it logical to live each day as if it may be our last? It would not be conducive to the running of a functional society, but nuts to that. One could argue that we must weigh the probability of death or non-existence in the near future and enjoying the present against the probability of it at a much later date and the value of planning and laying the groundwork for things to come. The problem is that it is impossible to compute these probabilities. We can attempt to guess at them, but we are very likely to underestimate the probability of death thanks to the "Black Swan" problem.

Suppose you just bought a lottery ticket for a drawing tonight. There's not much chance that you'll win, but you could. There is a chance. So would it be rational for you to act, right now, as if you're going to win? Obviously not. Likewise, it doesn't seem rational for you to act, right now, as if you're going to die. What would it mean to act that way? No need to study -- who cares about my future career prospects? No need to show up at work -- who cares if I get fired? No dentist appointments -- who cares if my tooth is going to rot, eventually? No need to save money, no need to go buy food for tomorrow, no need to answer all those emails in my inbox (that last one sounds esepcially tempting at the moment). Caring about oneself means caring about oneself as a whole, a person who exists through time, and thus if you have no special reason to think that you're going to die today, it would seem to be rational to care about one's life as a whole. That's not to say one shouldn't take...

# I am going under anesthesia in about a month. Once it is administered and I am unconscious, how do I know that the person who wakes up will be me and not a doppelganger with my memories? In other words, how do I know my stream of consciousness will continue after a period of unconsciousness instead of a distinct stream of consciousness starting for the first time?

The prospect of going under anesthesia is a scary one, for all sorts of reasons. But I don't think you should have much cause to worry about identity issues. I have two comments that might help alleviate your concerns. First, you might ask yourself: What would be the difference between its being you who wakes up from the anesthesia and its being a doppelganger with your memories? From the outside, you would seem exactly the same. And from the inside, it would seem the same too. Your doppelganger might be thinking something like this: "Yesterday I was worried about whether I would wake up from the anesthesia, and I'm glad that my worries were for naught -- here I am." In other words, the prospect that you're proposing is not really one that can be discerned -- either from the inside or from the outside -- as one that makes any difference to anything. But if that doesn't help (and I'm a bit worried that it won't), it might better reassure you if you think about anesthesia on the...

# However hard I try, I cannot shrug off the impression that philosophy asks all the truly important questions, but has always been somewhat vague when it comes to giving staightforward answers to those very questions. Do people have to turn to religion to get final answers? Because one thing is for sure: they are looking for those final answers.

Beware of straightforward answers to questions -- otherwise, you may end up like the folks who built the computer Deep Thought to answer the question of Life, the Universe, and Everything. When Deep Thought delivered the answer ("42") after 7.5 million years, it was a very straightforward one. But in this case it turned out that to understand the answer, you have to understand the question -- and no one really did. Getting clear on what the truly important questions are -- and how they should be understood -- is thus critical if we are to have any hope at answers that we can make sense of. So the question-asking role of philosophy is not one to be lightly dismissed. That said, I don't think it's true that philosophy fails to provide answers. You might be interested in Gary Gutting's recent book, What Philosophers Know , which attempts to show some answers that philosophy has provided in recent years.

# If everyone consistently uses a word wrong, does that eventually become the right way to use the word?

In thinking about your question, we might recall the conversation between Humpty Dumpty and Alice in Through the Looking Glass . At one point, Humpty Dumpty exclaims "There's glory for you." Alice protests that she doesn't know what he means. Humpty Dumpty smiled contemptuously. "Of course you don't -- till I tell you. I meant 'there's a nice knock-down argument for you!'" "But 'glory' doesn't mean 'a nice knock-down argument'," Alice objected. "When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean -- neither more nor less." "The question is," said Alice, "whether you can make words mean so many different things." "The question is," said Humpty Dumpty, "which is to be master -- that's all." Philosophers often distinguish between the semantic meaning of a word -- its assigned meaning in language -- and the speaker meaning of a word --what the speaker intends to mean by the word. Most of the time, if we want to...

# Does the brain contain the mind or does the mind extend beyond the brain?

Hmmm... did you submit your question via the new iPhone app by any chance? I ask because some philosophers have recently argued that the way that we use certain tools, like iPhones for example, extends our cognitive processing, and thus the mind, out into the world. Is there really a difference between the memories we store in our brains and the "memories" we store in external devices? When someone on the street asks you whether you know the time, you might answer affirmatively even though you have to consult your watch (or your phone) to tell them what time it is. You know the time in the sense that it is accessible to you. Likewise for the contacts that you have committed to iPhone memory rather than biological memory. Were someone to have a memory chip implanted in her brain, we might well accept that as part of the mind even though it is nonbiological. But why should it matter whether the chip is implanted in the brain. Couldn't it function the same way if it were outside the brain,...

# In the later 1700's, many famous philosophers (Locke, Berkeley, Hume) held the 'transparency thesis', the view that all important mental contents could only be conscious. Is this position still defensible?

You might want to take a look at some of the recent work of John Searle, such as The Rediscovery of the Mind . Searle argues there that "The notion of an unconscious mental states implies accessibility to consciousness. We have no notion of the unconscious except as that which is potentially conscious." (p. 152). While this isn't quite an endorsement of what you call the "transparency thesis," I believe it might be seen as a quasi-descendant of the view you describe. As an aside, the name "the transparency thesis" is nowadays often used by philosophers of mind to refer to quite a different phenomenon, namely, that when we introspect our experience, we don't seem to be able to attend directly to it; rather, our attention seems to slide right through to the objects of our experience.

# Hello philosophers, there is a vast philosophical literature that defends animal rights and vegetarianism, but the opposite camp doesn't seem to have produced much. What is the equivalent of Singer's "Animal Liberation" in the "meat eating" camp? Or is this a dead subject among philosophers, where those who care write books about the defense of animals, while those who don't simply go ahead and eat their steaks? Thanks in advance for your valuable insight.

I'm not sure that anyone has written the definitive treatise on Animal Enslavement , but you might check out The Animal Rights Debate . In this book, Tom Regan makes the case for animal rights, while Carl Cohen mounts a case against it.

# Is death without afterlife really all that bad? I mean, it could be worse, right? Of the plethora of possibilities the human mind can imagine, quiet, peaceful oblivion seems to me like not such a terrible thing.

I'm not sure that it's right to describe death without afterlife as "quiet peaceful oblivion." If there is no afterlife, and you cease to exist at death, then there is no you to experience the peace and quiet. If you've ceased to exist, then you have no experiences at all. It's precisely for this reason that philosophers like Epicurus claim that we should not fear death; on his view, all good and evil consists in sensation, and death is the absence of sensation. So, says Epicurus, "death, the most terrifying of ills, is nothing ot us, since so long as we exist, death is ot with us; but when death comes, then we do not exist." In short, if you're looking for a quiet, peaceful oblivion, I don't think death is the right place to look...

# Dear philosophers, I'd currently call myself a 'pseudo-vegetarian', in that I don't eat meat, but I do eat fish and dairy foods, and use other products derived from animals (e.g. leather, wool). I became a vegetarian when I was five; arguably, when it was easier for me to hold a black-and-white moral viewpoint. I would now like to re-evaluate my vegetarianism, so that I can make an informed and (hopefully) ethically coherent decision about the foods I eat and the products I use. Are there any books you could recommend for me to read? I studied some philosophy at university, and would be interested in reading some balanced discussions of animal rights, vegetarianism and veganism. Thank you for reading this e-mail, and thank you in advance for your help.

Given the difficulty in finding balanced treatments of these issues, you might try to achieve a balance of your own by reading people with strong opinions on either side of the issue. So in addition to reading Singer's Animal Liberation , which is the classic statement of the position in favor of animal rights, you might also read Peter Carruthers' The Animals Issue , which argues that animals do not have moral standing.