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Visual
illusions
such
as
the
Necker
Cube
or
the
Penrose
Impossible
Triangle
or
the
Hollow
Mask
illusion
demonstrate
that
the
'reality'
we
see
consists
of
constrained
models
constructed
in
the
brain.
The
Necker
Cube's
two-dimensional
pattern
of
lines
on
paper
is
compatible
with
two
alternative
constructions
of
a
three-dimensional
cube,
and
the
brain
adopts
the
two
models
in
turn:
the
alternation
is
palpable
and
its
frequency
can
even
be
measured.
The
Penrose
Triangle's
lines
on
paper
are
incompatible
with
any
real-world
object.
These
illusions
tease
the
brain's
model-construction
software,
thereby
revealing
its
existence.
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