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target that can be sensed directly. As we’ve seen, these kinds of problems can be solved using nothing more than some basic sensors, a means of movement, and some biological wiring that joins the two together. For a one-celled animal seeking sustenance in a lakebed, a sowbug on its way to the dark, moist underside of a rock, or even a basic robotic device, things can be just that simple. Though we humans share these basic elements with all other animals, our guidance mechanisms are embedded in a much larger and more complicated system. Our ceaselessly moving eyes perch atop a complicated tower of flesh, flicking from one viewpoint to another in an elegant dance that helps us to put together an overall view of the world. The basic rules that get us from the street corner to the bus stop, or from the kitchen table to the front door, may not differ substantially from those used by bacteria, insects, or othersimple beings, yet the detailed differences in how we use our senses to construct a sensory world will assume increasing importance as this story progresses.
    Many of the everyday challenges of space may involve nothing more than finding a way to move toward a target that is clearly visible, but this is hardly what we think of as wayfinding. More challenging and interesting tasks involve seeking out targets that cannot be seen directly. Here we enter a new realm where we find positions by using the relationships among things, rather than the very simple changes in the apparent size, shape, and strength of sensory signals that characterize our use of taxic mechanisms.

CHAPTER 2
LOOKING FOR LANDMARKS
H OW W E S EARCH FOR THE I NVISIBLE
BY U SING THE V ISIBLE
    The philosophy of the school was quite simple— the bright boys specialized in Latin, the not so bright in science and the rest managed with geography or the like
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    AARON KLUG
    O ne of the worst jobs I ever had was poring through old life insurance records to discover the names and birthdates of children of policyholders so that the company I worked for could create a computer program to send out birthday cards to those children. Though the job was staggeringly dull, there was one saving grace. The office I worked in was near the top of a skyscraper on the outskirts of downtown Toronto, and from its south-facing windows, I was able to watch the construction of the CN Tower, until recently the tallest free-standing structure in the world. The highest parts of this tower were built using a magnificent Sikorsky Skycranehelicopter, an undertaking of such significance that the schedule of appearances of the machine was published in some local newspapers and broadcast on the nightly news. I had a front-row seat, free of charge, provided I could master the art of pretending to fill computer coding sheets with names and dates while watching the tower take shape.
    As a young man whiling away his hours at a boring job, I had no sense of the transformative effect the tower would eventually have on the city. The main justification for the structure was that the boom in high-rise construction in downtown Toronto had begun to impede various kinds of radio telecommunications. But the rationale clearly had as much to do with establishing a “world-class” landmark for the city, an identifiable icon of space-age advancement, as it had to do with the pragmatics of transmitting radio waves and microwaves. But as well as serving as a landmark in the more colloquial sense of the word—as a structure whose silhouette has become identifiable as a part of the Toronto skyline as readily as New York City’s Empire State Building or Seattle’s Space Needle—the CN Tower has come to serve as a true landmark in the navigational sense. Wherever you are in the core of the city, or even in the outer fringes, it doesn’t take much of an effort to find the tower and thereby to help fix your own location. (The positional fix is helped along by the fact that the tower is located near the north