In daily life, however, we are
usually required to discriminate
among similar members of a
class. The infant must learn its
parents’ faces, the child must
learn to read, the zookeeper
must learn to differentiate the
monkeys, and so on. By dint of
necessity, then, whether or not
we are consciously aware of
doing so, we begin to attend to
the subtle nuances or details.
Naturally, when we do so, the
perception changes in the
direction of greater specificity
and precision. Once that
happens, we acquire a family of
slightly different memories
rather than merely the one
undifferentiated memory we 
had at the outset. We can then
make separate associations to 
each of these—–for example, a 
name. If at this point, when 
looking at twins, we notice 
that one has a wide nose and 
high cheekbones whereas the 
other has a narrower nose and 
lower cheekbones, we can say 
that we perceive the twins 
differently.

    If this reasoning is correct, 
attention is a very important 
factor in perception and we 
will return to this topic in 
more detail shortly. The claim 
is not merely that we can bring 
this or that object, all of which 
are perceived, into the center 
stage of awareness. The claim is 
that such a manipulation of 
attention affects perception 
itself.