Security is both a reality and a feeling. The reality of security is mathematical, based on the probability of different risks and the effectiveness of different countermeasures . . . But security is also a feeling, based on individual psychological reactions to both the risks and the countermeasures. And the two things are different: You can be secure even though you don’t feel secure, and you can feel secure even though you’re not really secure.
The essay embodies Schneier’s typically clear-eyed analysis of security issues. It is worth reading in full, either at the link above or at Schneier’s blog.