"When the blessed man was living for some days in the province of the Picts, he was obliged to cross the river Ness; and when he reached the bank of the river, he saw some of the inhabitants burying an unfortunate man, who… was a short time before seized, as he was swimming, and bitten most severely by a monster that lived in the water." Yes, you're right: it's the first recorded account of the Loch Ness monster, ascribed to this date in AD 565. The "blessed man," St.Columba, quickly subdued the beast using the sign of the Cross, but it has come back intermittently, particularly during the 20th century, to star in films, photographs, and sonar displays - baffling visitors and boosting tourist revenues for this beautiful but remote spot.
True, St. Columba was on the river Ness, a sea estuary where killer whales are not unknown. True, plenty of early medieval saints' legends feature quelling of water monsters; and almost all sightings of "Nessie" involve some degree of error, fraud, or essential fuzziness. What's interesting, though, is not the monster itself - it's our disappointment at the thought that it might not really be there.
Our favorite outdoor illusions - Nessie, Bigfoot, big cats, dragons - show that we are fundamentally naturalists, and that our brains have never really left the forest. Although our lives are increasingly dominated by the inanimate, from cars through computers to microwave ovens, their designers must still work hard to imbue all this dumb stuff with subtly biological characteristics - "happy" grilles, silky skin textures, chirpy alert tones, softly yielding buttons or shift-sticks - because our brains respond differently to animals than to other moving objects.
In a 2007 test, people shown pairs of slightly-differing photographs were much better able to spot the changes if they involved an animal than a van, even though the van occupied much more of the picture. We can infer animal motion from remarkably few clues: a shifting pattern of white dots on a black background. When these dots mark the joints of a walking person, the observer spots it immediately and can even specify the size, sex, and indeed mood of that person with surprising accuracy. Within the brain, the sight of moving people or animals activates non-visual responses that are not activated by, say, the sight of trees or water: memory, spatial perception, emotions. So nuanced is this response that the computer-animation industry has largely given up trying to mimic human motion with software physics engines, and instead makes cartoon characters credible through "motion capture:" filming an actor in a suit with white spots on it.
In a risky world, when we still roamed the jungle, it was wise to assume that every indistinct shape was an animate being and that it meant us no good. It would be suicide to maintain skepticism as the tiger creeps ever closer - so we bolster our assumptions to the point of certainty. Our brains, after all, are not machines for discerning truth, but for providing answers. It is better to take any decision than none, because it is better to be wrong than lunch.
So it is not necessarily a sign of schizophrenia to see figures in the shadowy woods or hear voices in the babbling stream. This is just our senses doing their job - inferring significance - if perhaps a little too well. When the illusion is broken and we see the truth, the world loses a little meaning for us. We laugh as the tension loosens, but deep down we are slightly disappointed. The quest for mysterious beasts is not a disreputable branch of zoology, but instead a fascinating and legitimate tool of psychology; it is on the brain's map that we should mark, "here be dragons."