The Perception of 
Environmental Orientation

What determines whether or 
not an object looks vertical, 
tilted, or horizontal in the 
world? One might think that it 
has something to do with the 
object’s orientation relative to 
other objects. For example, a 
picture on the wall may look 
tilted because it is not aligned 
with the wall’s vertical and 
horizontal coordinates. As we 
shall see, this proposition is 
true. But it leaves unexplained 
how we perceive the 
orientation of the coordinates 
of the wall. To what can we 
compare them? It also leaves 
unexplained how we can 
perceive the environmental 
orientation of a single object 
more or less correctly, even 
when it is the only object 
visible. There must be another 
source of information about 
orientation available to us.