This chapter will demonstrate how to classify documents based on their contents, a very practical application of machine intelligence and one that is becoming more widespread. Perhaps the most useful and well-known application of document filtering is the elimination of spam. A big problem with the wide availability of email and the extremely low cost of sending email messages is that anyone whose address gets into the wrong hands is likely to receive unsolicited commercial email messages, making it difficult for them to read the messages that are actually of interest.

The problem of spam does not just apply to email, of course. Web sites have gotten more interactive over time, soliciting comments from users or asking them to create original content, which has compounded the spam problem. Public message boards like Yahoo! Groups and Usenet have long been victims of postings that are unrelated to the board’s subject or that hawk dubious products. Blogs and Wikis are now experiencing the same problem. When building an application that allows the general public to contribute, you should always have a strategy for dealing with spam.