Bactra Review    Quine's Mathematical Logic
Consider, by way of analogy, links on Web pages. Some particularly inane pages contain links to themselves. One could write a page linking to all pages which link to themselves, and this page could --- and, geek nature being what it is, almost certainly would --- link to itself. One could also draw up the (much, much longer) page linking to all pages which do not link to themselves. Or rather one could try: but if it does link to itself, it shouldn't, and if it doesn't, then it ought. (Stolen from an old Nova special on mathematics, which used the example of library catalogs.)