The original review contained a silly slip which I have fixed here (serves me right for hacking the text about last thing at night before submitting it. If you want to spot the error - go to the original here).
Do you remember having a rather disturbed night's sleep about a month ago? That was the night I stole your brain. After landing my flying saucer in your garden, I crept into your bedroom and surgically removed your sleeping brain. I whisked it to my laboratory back on Pluto and connected it up to a supercomputer running a virtual-Earth program. This computer is currently feeding into your brain the same patterns of electrical stimulation that used to be produced by your sense organs, when you still had some. So it seems to you as though you're still on Earth. But everything you seem to observe around you, including this newspaper, is actually virtual. You've been brain-snatched.
How can you tell this hasn't happened: that what you're experiencing now isn't virtual? It seems you can't. But if you can't tell whether this newspaper is real or virtual, then how can you be said to know it's real? This is a famous philosophical thought experiment. In just a few sentences, it seems to demolish something we would ordinarily take entirely for granted: our knowledge of the world around us. Thought experiments can induce an overwhelming sense of intellectual vertigo. What we thought was the firm ground beneath our feet suddenly crumbles and we're left dangling over a void.
Some of the most famous arguments and problems in philosophy are based around thought experiments. Bizarre stories about brain-transplants, runaway trams, concrete sheep and invisible gardeners abound. In The Pig That Wants to Be Eaten, Julian Baggini has collected together 100 entertaining examples. The format is essentially the same as that first successfully introduced by Martin Cohen's 101 Philosophy Problems. Each thought experiment is set up in one or two paragraphs, followed by a few hundred words of thought-provoking discussion. Baggini offers us a tempting smorgasbord of some of the most baffling, weird and occasionally downright creepy scenarios ever envisaged.
Not every example is taken from the world of philosophy. The story of the pig that wants to be eaten is based on Douglas Adams's talking cow in The Restaurant at the End of the Universe, a beast that presents itself to diners as the main course before parting with "A very wise choice, sir, if I may say so ... I'll just nip off and shoot myself." Would there be anything morally wrong with killing and eating an animal genetically engineered to want to be eaten? This is certainly an intriguing question. As Baggini points out, the mere fact that most of us find the idea of killing and eating such an animal revolting doesn't establish that we would be morally wrong to do so.
A word of caution. First-year philosophy undergraduates often fail to see the point of thought experiments. "How can such fanciful stories reveal anything of importance?" they ask. "After all, there are no talking pigs, are there?" Well, if one of the aims of philosophy is to establish what is true in principle, as opposed to what's merely true as a matter of fact (that's supposedly the job of empirical science), then even a merely possible counterexample will do. Suppose I claim that the only reason it's wrong to kill and eat pigs, and animals generally, is that they don't want to be killed and eaten. If you can come up with a hypothetical animal that wants to be killed and eaten, but that it would still clearly be wrong to eat, then you have refuted my claim. Whether or not any such animal actually exists is irrelevant to its effectiveness as a counterexample.
Still, there are reasons to be cautious about thought experiments. They often appeal to our philosophical "intuitions", to what it "feels right" to say about the situation described. But intuition can be a fickle thing. The philosopher Daniel Dennett suggests that some thought-experiments are little more than "intuition pumps". Indeed, by subtly changing the spin on the story, you can sometimes elicit quite different intuitions. But even while you ponder to what extent your own philosophical intuitions are to be trusted, you can still enjoy these mind-boggling tales from the outer limits of thought.