A second reason is that lawful 
processes of perception must 
have evolved to allow us to 
achieve veridicality. An animal 
can hardly be expected to find 
out about what is actually 
present or happening in the 
world by asking someone or by 
reading a book. Nor can it wait 
to find out by exploring the 
situation. Perception must be 
capable of yielding accurate 
information immediately and 
through the senses. It is 
understandable that the laws of 
perception that allow us to gain 
such information will also, 
under certain conditions, lead 
to misperception. We can 
hardly expect such lawful 
processes to be suspended 
simply because we find out 
that, for example, two lines 
that we perceive to be unequal 
in length are in fact equal or 
that the moon, which we 
perceive to be larger at the 
horizon, is in fact equal in size 
and distance from us whatever 
its position in the night sky.

    Still another reason for the 
insulation of perception from 
conceptually based knowledge 
may be this: Perception is 
sometimes based on inference 
from rules, but the rules seem 
to be "known" on an 
unconscious level. For 
example, it is probable that the 
perceptual system "knows" the 
law of the visual angle, that 
the visual angle is inversely 
proportional to an object’s 
distance. This law is invoked 
and employed unconsciously 
when it enters into the 
processing of size at some 
distance. Therefore, the 
process underlying perception 
will run its course on the basis 
of unconscious "knowledge"; it 
is thus not influenced by 
consciously available 
information. Consequently, 
when one inspects one’s own 
afterimage projected on 
surfaces at differing distances, 
it matters little that one 
consciously knows it is an 
image of unchanging size. It 
will still appear to change its 
size because of the unconscious 
application of the corollary of 
the law of the visual angle, 
Emmert’s law, that the size of 
objects yielding a constant 
visual angle is directly 
proportional to their distance.