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5 Illusion, delusion, collusion, and
perceptual paradox

The pictorial effects we have been discussing all fall into
the broad category of illusion. It is the purpose of this
chapter to shed some light on the experience one can have
when confronted with objects that fall under this rubric.
The Oxford English Dictionary defines “illusion” as follows:

[ILLUSTRATION]

Figure 5-1. Stare at this square for
about a minute in order to observe
an afterimage. If you look at a distant
wall after impressing afterimage
on your retina, image will appear to
be larger than if you look at a surface
much closer to you.

One of the best-known examples of such a perception is
called the moon illusion, the impression that the moon is
larger when it is close to the horizon than when it is close
to the zenith. Lloyd Kaufman and Irvin Rock confirmed
in 1962 a theory that has been attributed to Ptolemy,[1] to
wit, that the moon appears larger on the horizon than at
the zenith because the filled space between the observer
and the horizon makes the horizon seem further than the
zenith[2] (Kaufman and Rock, 1962; Rock and Kaufman,
1962). There is an implicit inference here that is based on
the following law: All other things being equal, the further
away an object (of constant angular subtense) seems to be, the
larger it will appear to be.

An especially pure example of the operation of this law
was discovered by Emmert, in 1881. It is also easy to
demonstrate. Look at the black square in Figure 5-1 for
about a minute. When you look away, you will see a dark
spot in front of you; this dark spot moves as you move
your eyes, because it is caused by the neurochemical process
by which the photosensitive cells in your retina recover
from the unusually prolonged exposure that they sustained.
Because this effect is impressed on the tissue of the
retina itself, it must move with your eyes. At first blush,
it may seem surprising that such a purely internal activity
feels as if it were located outside you; but that is a general

rule in perceptual systems: If one stimulates sensory receptors
in a nonstandard fashion, one invariably experiences
an external object that would stimulate the sensory
receptors in a similar fashion. Now this sort of effect on
the retina could just as well have been caused by a distant
large square or by a close small one. Because an afterimage
does not, so to speak, remember the distance of the page
on which the stimulating square was printed, the size and
distance of the black square that one experiences when
having an afterimage would remain indeterminate were it
not that perceptual systems abhor indeterminacy. (Try to
think of what a square of indeterminate size and distance
would look like.) To forestall such indeterminacy, the visual
system uses the best available information about the
size and the distance of the square: It assesses the distance
of the surface at which the observer is currently looking,
and, using that information and information about the size
of the afterimage on the retina, it computes the size of the
square to be seen. So if — after you have impressed an
afterimage on the retina — you look at a distant wall, the
square will look large; and if you look at a sheet of paper
that is close to you, the square will look small. We can
now state Emmert's law: The apparent size of the object you
see when you experience an afterimage is directly proportional to
the perceived distance of the surface at which you are looking.

The moon illusion and Emmert's law are both examples
of an important way in which perceptual systems are endowed
with the ability to perform what Helmholtz[3] called
unconscious inferences, an idea that is central to what I wish
to say about illusion and art in this chapter.[4]

Do we ever use the term “illusion” in the sense that
applies to the moon illusion when we apply it to art? I
think not: I do not think there ever is “false belief or conception”
when we look at a work of art. Arthur C. Danto's
discussion of illusion (in the sense of false belief or conception)
shows clearly why we should hold this view:

[ILLUSTRATION]

Figure 5-2. A classification of
trompe l'oeil pictures

I take it for granted that the reader agrees with Danto's
claim that the artwork should not be reduced to its content,

or else that he or she will read his persuasive argument in
Chapter 6 of The Transfiguration of the Commonplace.

The only works of art that come close to exemplifying
this sort of illusion are the illusionistic architectures we
discussed in the preceding chapter and trompe l'oeil paintings.
To better understand the role of illusion in art, let us
examine this interesting aberration of art. I have classified
the illusionistic paintings that go under the name trompe
l'oeil (eye foolers) in Figure 5-2. The pictures fall into two
major groups according to what the artist has represented.

A trompe l'oeil painting of the first kind looks like a
painting; a delusory representation is superimposed on a
painting that is taken by the viewer to be just that — a
painting. I group these paintings under the rubric of extrinsic
trompe l'oeil. There are two subgroups in this class.
First, there are paintings in which an element foreign to
the painting is painted to look like a foreign element. For
instance, Carlo Crivelli's Saints Catherine of Alexandria and
Mary Magdalene (see Figure 5-3), shows a fly on the left
side of the left-hand niche.[5] We may say that such paintings
are trompe l'oeil of an adventitious element (e.g., the fly).
The second sort of extrinsic trompe l'oeil is a play on the
viewer's expectations regarding the frame or framing elements.[6]
For example, Antonello da Messina, in his Salvatore
Mundi (Figure 5-4), painted a cartellino (little card),
a trompe l'oeil representation of a creased piece of parchment
bearing an inscription. As Marie-Louise d'Otrange
Mastai (1975) has pointed out, Antonello's use of the cartellino
is in keeping with the earlier device used by portrait
painters: Sometimes they would paint an incised inscription
on the parapet or sill in the foreground that creates
the impression that the subject of the portrait is very close
to the picture plane. An example is Jan van Eyck's Portrait
of a Young Man (Figure 5-5). Eventually, when the parapet

[ILLUSTRATION]

Figure 5-3. Carlo Crivelli (attrib.),
Saints Catherine of Alexandria
and Mary Magdalene (1480–5).
The National Gallery, London.

was abandoned, whenever the cartellino was retained, it
became more thoroughly trompe l'oeil by appearing to be
pasted on the surface of the painting itself. One such case
is Francisco de Zurbarán's Saint Francis (Figure 5-6). Another
use of framing elements for the purposes of trompe
l'oeil is the representation of a broken glass in front of the
painting. An example is a painting by Laurent Dabos (Figure
5-7).

The second class of trompe l'oeil paintings, if successful,
are not read as paintings at all. I consider them instances
of intrinsic trompe l'oeil. They fall into three categories: (1)

[ILLUSTRATION]

Figure 5-4. Antonello da Messina,
Salvatore Mundi (1465). The
National Gallery, London.

simulated texture or relief, (2) simulated objects or settings,
and (3) display boards.

To simulate a bas relief or a texture, one needs for the
most part to work in monochrome. When gray stone is
to be simulated, the technique is called grisaille (the term
comes from gris, the French for gray). If the material is
not gray — such as bronze, terra-cotta, onyx, marble, or
wood — a trompe l'oeil painting that simulates any of them
is called camaïeu.[7] Figure 5-8 shows an example of this
technique.

There are three sorts of trompe l'oeil objects and settings:
(a) cutouts, (b) hearth screens, and (c) objects painted on
odd surfaces. Chantourné (literally, cutout), is a trompe
l'oeil representation designed to stand away from a wall.
An example is Cornelis Gijsbrechts's Easel (Figure 5-9).[8]

[ILLUSTRATION]

Figure 5-6. Francisco de Zurbarán,
Saint Francis in Meditation
(1639). The National Gallery,
London.

The effectiveness of chantourné paintings relies on an
impression of solidity derived from the shadows they cast
on the walls behind them. Often, as in the case of Easel
the chantourné includes a painting, usually a skillfully illusionistic
one. The hearth screen, devant de cheminée, a
French invention, was quite popular during the late seventeenth
and the eighteenth centuries. This type of painting
fools the eye because we do not expect a screen there, and
whatever is represented is mundane and does not violate
our expectations regarding what we might find in an unused
hearth during the summer. The objects are strongly
illuminated in the foreground and quite dim in the background,
where the niche of the hearth casts a shadow. Even
Jean-Baptiste Chardin painted one (Figure 5-10). If the
hearth screen is designed to disguise the existence of the
surface on which it is painted, there is a similar trompe
l'oeil effect that can be obtained by painting on a surface

[ILLUSTRATION]

Figure 5-7. Laurent Dabos, Peace
Treaty between France and Spain
(after 1801). Musée Marmottan,
Paris.

[ILLUSTRATION]

Figure 5-8. French School, Rome
(early nineteenth century). Cooper-Hewitt
Museum, New York.

[ILLUSTRATION]

Figure 5-9. Cornelis Gijsbrechts,
Easel (1633). Statens Museum for
Kunst, Copenhagen.

that is an unlikely candidate to play such a role. An example
is van der Vaart's Painted Violin (Figure 5-11).

We finally come to the best-known class of trompe l'oeil
paintings — the several types of display boards. For example:
Figure 5-12, the hunting trophy; Figure 5-13, the
quod libet (what you will), which eventually evolves into
the letter-rack; Figure 5-14, the vide poche (pocket emptier);
and Figure 5-15, the poster board.

[ILLUSTRATION]

Figure 5-11. J. van der Vaart (attrib.),
Painted Violin (late seventeenth
or early eighteenth century).
Devonshire Collection, Chatsworth,
England.

Although it would take us too far afield to engage in an
analysis of the significance and psychological bases of these
trompe l'oeil works, I do want to point out the role of
attention and expectation in creating the delusions to which
these works can give rise. John Kennedy has taken a first
step toward elucidating the role of attention in trompe l'oeil
phenomena. He asked children to add a drawing of a figure
in the midst of the children shown in Figure 5-16. When
they concentrated on the central region of the picture, many
of them absentmindedly tried to pick up the pencil. This
observation suggests that although the standard claim about
trompe l'oeil — namely that it requires the representation
of an object of shallow depth — is true enough, it fails to
do justice to the psychological complexity of the phenomenon.
It is perhaps correct as a statement of a necessary
condition for the occurrence of the trompe l'oeil effect,
but it leaves the question of the effect's sufficient conditions
unasked.[9]

What is it about the delusion of trompe l'oeil that makes
such works interesting? After all, there is nothing fascinating
in a trompe l'oeil painting until the delusion has
been dispelled; and once it has been dispelled, the work is
most often of no more than minor esthetic interest. We
enjoy examining an object endowed with the power to
throw us into a delusory state of mind after it has divulged
its secret to us; looking at it sends a shiver down our
metaphysical spines much in the way we shiver when we
think about an accident in which we were almost involved;
we stare at it much as we might stare at the carcass of a
wild animal that almost got the better of us. A trompe
l'oeil picture is an epistemological close call, a reminder
that Descartes's evil being that continuously fills us with
error may be disguised as a benevolent painter. The point
I wish to make therefore is that what is interesting about

[ILLUSTRATION]

Figure 5-13. Edward Collier,
Quod Libet (1701). Victoria and
Albert Museum, London.

a trompe l'oeil painting arises in our minds after the painting
has ceased to trompe our yeux; it is when we have ceased
to be the unwitting targets of a practical joke, and we have
decided to reflect upon the experience we have just gone
through, that the painting acquires its meaning.

And then looking at a trompe l'oeil painting after the
delusion has been dispelled is fascinating because it shows
us how utterly preposterous was Ruskin's famous idea of
the “innocent eye.” One tries in vain to be deluded again,
but one can't; at best we are impressed by an illusion, which
we obtain by actively cooperating with the artifices devised
by the artist. But there is always a sense of innocence lost,
a banishment from paradise, a fool's paradise to be sure,
but paradise nevertheless.

All illusionistic art other than trompe l'oeil relies for its
effect on a collusion between the artist and the spectator.

Consider illusionistic paintings of architecture for a moment.
None of these paintings places the spectator at the
center of projection at the moment the picture becomes
visible. For instance, Pozzo's imaginary architecture in the
Church of Sant'Ignazio looks lopsided unless it is seen from
the yellow marble disk in the center of the church's nave:
Therefore, only a visitor who would have asked to be led
blindfolded to the prescribed vantage point would see the
painting correctly, as it were, at first sight; but to have
prepared one's experience so carefully presupposes prior
knowledge of the spectacle one was about to behold and
enjoy. Most viewers deeply enjoy the experience despite
having first seen it lopsided and distorted. These viewers

[ILLUSTRATION]

Figure 5-14. Samuel van Hoogstraten,
Still Life (1655). Gemäldegalerie
der Akademie der bildended
Künste, Vienna.

[ILLUSTRATION]

Figure 5-15. Wallerant Vaillant,
Four Sides (mid-seventeenth century).
Galleria Lorenzelli, Bergamo,
Italy.

are in mental collusion with the artist who designed and
painted the illusionistic architecture because they know full
well that they are experiencing an illusion when they view
the ceiling from the center of projection.

This concept of mental collusion is similar to Coleridge's
“willing suspension of disbelief for the moment, which
constitutes poetic faith” (1907, Book II, Chapter 14, p. 6).
The difference is one of degree: Willing suspension of
disbelief refers to a cognitive operation, a voluntary adoption
of a certain aesthetic attitude; by mental collusion with
the artist, I mean an operation much closer to the roots of
perception, more on the order of a suggestion than a frame
of mind.

The concept of mental collusion appears in nonaesthetic
perceptual contexts as well. For instance, certain illusions
do not occur spontaneously or involuntarily; they occur

[ILLUSTRATION]

Figure 5-16. Drawing used by
Kennedy

only after the viewer is informed what he or she is expected
to see. But once that knowledge is imparted, there is little
the viewer can do to escape its effect. As an example,
consider the experiment in which Girgus, Rock, and Egatz
(1977) measured the time it took observers to experience
a figure — ground reversal in Rubin's (1915) vase — face figure
(see Figure 5-17), which was thought to spontaneously
reverse back and forth between the vase percept and the
face percept. The observers were high-school students who
had never seen the Rubin figure before. Every 5 seconds,
the experimenter tapped a pencil to mark the moment at
which the observer was to report what he or she was seeing
in the figure. Every effort was made to communicate to
the observers that certain figures could be described in
more than one way, and that therefore their reports could
differ from signal to signal, but they were not told that
the Rubin figure was reversible and they were not told
what the alternative descriptions could be. After having
obtained the observers' reports, the experimenter interviewed
them to ascertain whether unreported reversals had
occurred at every tap. Even with this scoring procedure,
which was most likely to overestimate the number of reversals
seen spontaneously, only 50 percent of the observers
saw the figure reverse within the first minute of viewing,
a figure that went up to 60 percent within the first two
minutes and to 65 percent within the first three minutes.
During the interview, observers were taught to see both
alternatives and to grasp the reversibility of the figure.
Afterward, the observers were tested again and, as expected,
all of them reported reversals.

[ILLUSTRATION]

Figure 5-17. The vase-face reversible
figure.

To better clarify the notion of mental collusion, let us
look at the wonderful illusion invented by Bradley, Dumais,
and Petry (1976; see Figure 5-18). The initial impression
one receives is of a white paper cutout of a Necker
cube superimposed on a sheet of white paper on which
eight black disks have been drawn in order to enable you
to see the figure's critical features. Even though there are
no lines joining the corners, you see them, an unconscious
inference regarding the nature of the object that would
create this sort of configuration. You are not free to see

[ILLUSTRATION]

Figure 5-18. A Necker cube formed
by cognitive contours as a perceptual
analog of willing suspension of
disbelief

or not to see these cognitive contours: If you see the Necker
cube as I described it, you always see the contours. When
you do, you also can see the cutout as a representation of
a three-dimensional object, and, because the representation
is ambiguous, you can see it reverse, as does the Necker
cube. Now the interesting twist to this illusion comes when
one's attention is drawn to another way of interpreting the
eight spots. Imagine a sheet of paper with eight holes in
it, and under it a sheet of black paper that can be seen
through the holes. Now suppose we took the white paper
cutout of the Necker cube and slipped it between these
two sheets so that the critical features were visible through
the eight portholes in the top white sheet of paper. When
you interpret the figure in this fashion, you can still “see”
the Necker cube, and you can still experience reversals of
its orientation, but you do not see the cognitive contours. The
act of choosing to see the cutout of the cube behind a page
with holes in it rather than in front of the page with spots
on it is very much like a willing suspension of disbelief.
But once one has made a commitment to that suspension
of disbelief, the world we perceive is consistent with how
we have chosen to perceive it. It is important to remember
that we are not in a position to reinterpret every facet of
our perceptual experience and to see how the implications
of our choice propagate through the remainder of our experience.
But there are certain aspects of experience that
allow us to make such a choice, although, unfortunately,
we do not understand what gives them this power.[10]

We appreciate illusionistic art without being deluded;
we know that what we are seeing is mere artifice; we
experience illusion because we are in collusion with the
artist. In contrast to illusionistic art, we appreciate trompe
l'oeil because we were initially deluded. Mental collusion
has very little to do with our appreciation of these creations,
which, if we appreciate them at all, are reminders
of the fallibility of knowledge acquired through the senses.

Having discussed the nature of delusion in trompe l'oeil
and the nature of collusion in illusionism, we turn now to
a third anomalous state of mind we sometimes experience
when viewing a painting, namely, perceptual paradox. In
the preceding chapter, we discussed the sorts of deformations
we perceive in paintings despite the fact that in
general perspective is robust. Although it seems paradoxical
that, at one and the same time as one passes in front
of a painting, the scene appears to turn and to remain the
same, it is possible because not all aspects of our perception
are processed by the same mechanisms; there is a division
of labor that usually works so well that it is not noticed.
The well-trained bureaucracy of the mind can deal with
practically all the contingencies that occur in our environment.
But when psychologists contrive devices that stimulate
us in unusual ways, ways that are unlikely to arise
in our environment, perception can be made to reveal the
division of labor without which it could not function. The
rules by which the bureaucracy has been accustomed to
work may now lead to incompatible decisions.

For instance, take the waterfall illusion: On a screen, we
display an unbroken series of horizontal black stripes moving
downward. After a viewer stares at this stylized waterfall
for a while, the motion is stopped, and he or she is
asked to report what the display looks like. The display
looks paradoxical: The stripes appear to be moving upward,
but at the same time each stripe does not seem to
be changing its position relative to the frame of the screen.
This sort of perceptual decomposition has led to the hypothesis,
now well-supported by experimental evidence,
that motion and location in space are processed by different
mechanisms (Attneave, 1974). No less interesting, though,

[ILLUSTRATION]

Figure 5-19. The vertical — horizontal
illusion

is the following implication of the phenomenon: The visual
system makes no attempt to reconcile these contradictory
pieces of information about the world; we experience these
unreconciled contradictories, this perceptual paradox, as
illusion.

It is important to keep in mind the distinction between
illusion as perceptual error, which we have called delusion,
and illusion as an awareness of perceptual error, which we
have called collusion. As we have seen in our discussion
of the vase — face illusion, most illusions do not provide us
with the experience of illusion unless we are given an opportunity
for collusion, an understanding of what we are
to expect to experience. Take, for instance, the vertical-horizontal
illusion (Figure 5-19). The vertical looks longer
than the horizontal: That is a perceptual error. But it is
only when you are put in a position to experience a perceptual
dilemma — such as being told to rotate the drawing
slowly, and becoming aware of the changes in the relative
lengths of the two lines during the rotation, while realizing
that the drawing itself is invariant — that you may experience
an illusion. This is a metaperceptual experience: It is
an awareness of perceptions; the visual system does not
try to reconcile the two experiences, and that nonreconciliation
gives rise to the experience of illusion.

The impression of following in a painting is one of those
rare instances where an object spontaneously gives rise to
the experience of an illusion. My explanation of this phenomenon
is schematically summarized in Figure 5-20. The
experience of the picture turning stems from two perceptions:
On the one hand, even though we are walking around
the picture, we perceive the spatial layout of the represented
scene as if it remains unchanged. This is what we
have called in Chapter 4 the robustness of perspective (which
we will discuss at length later). On the other hand, even
though the spatial layout of the scene remains unchanged,
we perceive our own motion in space as we walk past the
picture. The experience of rotation of the painting is one
way to resolve this dilemma: To perceive the scene as being
invariant while we are walking past it, we must perceive
the picture to be rotating.

As Gombrich has pointed out,[11] this resolution of the
dilemma is reinforced in paintings that contain objects with
a pronounced aspect such as a foreshortened gunbarrel, a
pointing finger, a human eye, or a road receding into the
distance from the center foreground to the horizon (such
as the Rousseau painting discussed toward the end of the
preceding chapter). These are objects that are represented
in an orientation that is visually unstable: If you are looking
down the barrel of a gun, you need to take only a very
small step sideways in order not to be looking down the
barrel of the gun. We say here that objects are represented
in a visually unstable orientation by analogy with objects
that are in a physically unstable equilibrium, such as a
pyramid that has been balanced on its tip: You need to
apply only a minuscule change to the forces exerted upon

the pyramid to cause it to fall.[12] It is quite natural, therefore,
that we perform the unconscious inference: The object is
shown in a visually unstable orientation; I am moving
enough to destabilize the view; the view is not destabilized;
therefore, the object must be turning to follow me.

But that solution to the dilemma is, so to speak, shortsighted,
because it gives rise to another dilemma: If the
picture is turning, how is it that it looks so well attached
to the wall? Why does its relation to the room not change?
The experience of illusion stems from the visual system's
inability to resolve this dilemma within a dilemma.

Although we have shown that some distortions do take
place in the perception of paintings that are viewed by
moving observers, it is the robustness of perspective that
emerges most clearly from our analysis. As we will see
presently, it is this robustness that is probably the most
important justification for not using Brunelleschi peepholes
to view perspective paintings.