Two of my recent posts dealt with the question of whether Christine O'Donnell, currently the republican nominee for senate in Delaware, is a nut. That raises a more general question of some interest: What counts as nutty?Pretty clearly, it isn't enough to merely hold mistaken beliefs; we don't regard Ptolemy as a nut, although he famously believed that the earth was at the center of the universe, embedded in a nested collection of crystaline spheres. Those of us who are atheists do not conclude that all the religious believers are nuts although, seen from our perspective, the beliefs of many of them do indeed look pretty weird. So what does it take?In the first of my two posts, I mentioned that O'Donnell, in arguing that masturbation was sinful, was correctly (so far as I could tell) reporting a position expressed in the New Testament (not about masturbation directly but about lust), and that although I disagreed I did not take that as evidence that she was a nut. Several people commenting wanted to know why, or else clearly disagreed.In that particular case, I think there are two answers. The first is that I don't have any solid basis for my own moral beliefs, any way of proving to a reasonable and open minded skeptic that they are correct. That puts me in a poor position to condemn as obvious nonsense someone else's moral beliefs.I could have gone on to point out that a number of moral beliefs strongly held by many people in our current society would be seen as distinctly nutty by most people, including most intelligent, educated, and reasonable people, in a fair number of past societies. Consider, as one example, our rejection of slavery, contrasted with the view of the subject held by Aristotle and his contemporaries. For another, consider our view of the minimum age for sexual intercourse, reflected in age of consent and statutory rape laws, contrasted with the views of most past societies. There is good reason to believe that we know more about science than people did in the past, but I have not yet seen any evidence that we know more about moral philosophy. And I know of at least one twentieth century case where a legal change replacing traditional with modern views of the subject—in Jewish law by rabbis in Palestine—was defended by the factual claim that bearing children young was more dangerous now than it had been two thousand years ago, a "scientific" claim that strikes me as distinctly nutty.But the special problems of moral beliefs don't answer the more general question. There are lots of people who disagree with me on factual questions whom I don't consider nutty either. So where do I, where should I, draw the line.Consider a second and more troubling piece of evidence against O'Donnell. In a TV interview a few years back, she said:"American scientific companies are cross-breeding humans and animals and coming up with mice with fully functioning human brains."The story I got the quote from suggested that she was "misremembering this 2005 report on scientists who successfully grew human brainwithin mice."Misremembering the details of a news story doesn't qualify as nutty—people do that all the time. Getting facts wrong, even badly wrong, doesn't qualify either. As I pointed out in a piece some time back, David Frum, a prominent conservative commentator, got the facts about the beginning of the Great Depression backwards, thought Hoover cut spending when in fact he sharply increased it; that doesn't make him a nut. Biden, in his debate with Palin, described FDR as responding to the stockmarket crash by going on television to reassure the nation. That doesn't make him a nut either.What is disturbing about the O'Donnell quote is not that she got the facts wrong but that she got them so very wrong, believed something that, on the face of it, no reasonable person would believe. That strikes me as a good first cut at a definition of nutty.But the application is not as simple as it might seem, even in that case, because it depends on what one can expect a reasonable person to already know. Biden's comment contains two claims that, to anyone moderately familiar with American history, should have been obviously false—that FDR was president in 1929 and that television was widely available at the time. But most Americans are not even moderately familiar with American history, and that probably includes quite a lot of prominent American politicians.Similarly with the O'Donnell quote. The idea that one could get a fully functional human brain into a mouse's skull is, on the face of it, absurd—to someone with any feel for either the current state of biotech or the relevant biology. But I think there is quite a lot of evidence—most obviously the circulation figures of the wilder tabloids—that a sizable fraction of the American population doesn't have any feel for that sort of thing. Do they all count as nuts?Let me go back to the question of the Bible, not as a source of moral authority but as a source of truth. My first instinct is to write off anyone who believes that as obviously crazy. And I am at least tempted to broaden that to anyone who believes the central claims of any of the major religions, anyone who, in Orwell's phrase, believes in Heaven the way he believes in Australia.The reason I don't write them off that way is that I know of too many people, present and past, who quite obviously were intelligent, thoughtful, and reasonable, yet believed in the Bible, some of them in a pretty literal sense, others at least to the extent of believing in its central claims.In trying to make sense of all this, I fall back on the observation that most of what most of us, perhaps all of us, believe, is based not on evidence directly available to us but on what the people around us tell us. Not only is it so based, it has to be. Nobody has the time and energy to check enough of the facts for himself—to be sure that Australia, and New Zealand, and Antarctica, and Orford, N.H., actually exist by going and looking at them, rather than by believing what he is told or reads.One reason I am confident it can't be done is that I know someone who tries, a fellow poster to Usenet with whom I have interacted over a period of many years. He is an intelligent and energetic fellow, and he appears to follow a policy of regarding with skepticism anything he can't check for himself. Thus, for example, he takes it for granted that none of the official figures on inflation can be trusted, and tries to make his own estimate from prices he has himself observed. His conclusions, in that case and many others, sharply diverge from what the rest of us believe. One result is that he comes across, to many people and not entirely without reason, as a nut.Once you accept the practical necessity of relying heavily on second hand information, you have to modify your view of what a reasonable person would believe to take account of what those around him believed. If you have no training in science and your only information on biotech comes from the popular press, it may not be obvious that a story on mice with human brains cannot be right. If you have devoted your time, energy, and intelligence to living your own life, doing your job, dealing with those around you, it isn't all that unreasonable to accept as truth what those around you believe about wider issues less directly observed, such as the existence of God or the weakness of the case for evolution.That applies not only to people in the past who couldn't have known the evidence for evolution but to people in the present who could have but in all probability don't. I long ago concluded that most people who say they do believe in evolution, like most who say they don't, are going mostly on faith. As I pointed out in a post some years back, many of those who say they believe in evolution, most notably people left of center, have no difficulty rejecting even its most obvious implications when those clash with their ideology.So what does qualify one as a nut? I think the best answer I can come up with is holding beliefs that no reasonable person with your intellectual background could hold. In practice, since one rarely knows enough about some else's background to apply that criterion, it comes down to observing how someone holds and defends his beliefs. Someone who argues for creationism and against evolution in a coherent, consistent, intelligent fashion isn't a nut, even if there are lots of facts he doesn't know that contradict his argument, even if he bases his attack on a mistaken (but widely believed) account of the contents of the theory he is attacking.It is at the point when the argument depends on ignoring facts he does know, on defending inconsistent positions, demonstrates that he is committed to the conclusion whatever the evidence and the arguments might be, that the balance begins to tip. The clue is not what he argues for but how he argues for it.At least, I think that's the closest I can come to answering the question that started this post.