Even though our perceptions
are mental constructions
rather than direct recordings of
reality, they clearly are
neither arbitrary nor mostly
illusory. Members of every
species must correctly
perceive, however differently,
certain aspects of the external
world. If they did not, they
would never be able to obtain
the necessities of life or to
avoid its dangers and would die.
We are no exceptions. Within
the range of stimuli to which
our senses are attuned, our
perceptions of the sizes,
shapes, orientations,
stabilities, and lightnesses of
things turn out to be not simply
different from the images
formed on our retinas but
remarkably correct, or, as
students of perception say,
veridical. Philosophers rightly
point out that we have no direct
access to what is "really" 
there in the world other than
through our senses. But by
veridical is only meant that our
perceptions correspond with
the properties of things
considered objectively and
independent of viewing
conditions, such as can be
ascertained by measurement.
Thus our perception of a shape
such as a circle can be said to
be veridical if we know or can
easily determine by
measurement that the object
has equal diameters in all
directions.