Optical illusion
The Fraser Spiral (seefigure) is an optical illusion: it is carefully con-
structed in such a way that it looks spiral; but, in fact, it is a series of
discrete, concentric circles, which do not touch each other. To begin
with, let us note some of the features of this illusion. First, the illusion is
not total–it is not, say, a very (or maximally) realistic dream. The latter
is the kind of‘illusion’suggested when philosophers speak of the‘argu-
ment from illusion’, which begins by imagining someone (say)completely
under the illusion (i.e. very convincingly dreaming) that she is in front of
the Taj Mahal.^40 It is a stipulated feature of the totalising illusion that
‘one cannot tell from the phenomenology of one’s awareness’whether or
not it is an illusion.^41 But the Fraser Spiral is not like that. It is clearly
distinguishable from a real spiral, because you can follow any individual
circle with a pencil and see that you are going round in a circle, not
spiralling into the centre. Indeed, one could go further: optical illusions
are often found in books for children, and it is often part of the fun of
optical illusions that the person under the illusion can show that the
picture on the page is not as it appears to be. Illusions like these may also
play a significant part in visual art: hence, a talented sketch artist can
make the white of the blank page in one part of the picture look brighter
than the same white page in another part of the picture.^42
A second feature of the Fraser Spiral has been alluded to already, but let
us be more explicit in this case. As an optical illusion, the Fraser Spiral
is‘belief-independent’:itlookslike a spiral to me, even once I know that
it is not a spiral. As I look at the Fraser Spiral, I am still experiencing
an illusion–even though I don’t have any false beliefs about it at all.
Truth and illusion 57