The change following or preceding a distinguished phenomenon must not be unique, but
share some properties with changes associated with similar phenomena. A minimal
regularity or invariance of effect is needed in order to make
predictions (Heylighen, 1989). The invariance criterion can be subdivided in a
number of more specific criteria, specifying under which transformations of
initial phenomena (causes) the characteristics of the effect will be invariant.
The simplest type of invariance is probably that over time
("consistency" according to Kelley, 1967): subsequent appearances of the
distinguished phenomenon should lead to similar effects. An effect can also be
invariant over settings or circumstances, or over different points of
view or modalities of perception. The more invariant the causal
relationship, the more generally reliable and applicable it is, the more
predictions can be made with it, and the more useful the corresponding piece of
knowledge is.
If the same external phenomenon is
perceived from different angles, distances or illuminations, it will appear
differently, yet maintain some constant distinction or identity. The sensation
produced e.g. by a a particle of dust in the eye, on the other hand, will not
change when the person moves, or will change in a random way, without relation
to the movement of the body.
According to
atribution theory people attribute causes of perceived effects to those phenomena
that covary with the effects, that is to say that are present when the effect
is present, and absent when the effect is absent. To determine whether a
perception is real, you should determine whether its cause is some objective,
external phenomenon, or some internal mechanism (e.g. imagination,
hallucination, malfunctioning of the perceptual apparatus).
Now, external or objective causes will not covary with changes that only
affect internal or subjective variables. For example, some aspects of a perception of an external objects, such as size, and
angle will covary with the movement of the perceiver (internal change). On the
other hand, the fact that something is perceived at all, should not vary with
the location of the observer: it should be invariant over positions. Kelley
(1967) has proposed the following set of criteria that characterize external or
"real" causes of perceptions:
- a. invariance over time (consistency)
- : a perception that appears or
disappears suddenly is unlikely to be caused by a stable object
- b. invariance over persons (consensus):
- a perception on which different
observers agree is more likely to be real than one that is only perceived by
one person.
- c. invariance over modalities:
- this is an extension of the point above.
If the same phenomenon is perceived in different ways, or through different
senses (e.g. sight and touch), it is more likely to objectively exist, rather
than to be the effect of a malfunctioning perceptual system.
Remark that none of the above criteria is sufficient on its own to establish
the reality of a perception. For example, a mass hallucination or a magician's
illusion may be perceived by thousand people, and hence satisfy criterion b.,
without being caused by an objective phenomenon. A malfunctioning of the eye or
of the nerves may be stable, and hence produce a continuing excitation of the
brain, satisfying criterion a. A hologram can be looked at from different
points of view (criterion c.), but does not correspond to a real object.
Yet, the perception will become more reliable or more real when the different
criteria are fulfilled to a larger degree. The hologram cannot be touched
(different modality), and that gives away its illusory character. The
magician's spectacle may be watched by another magician who would quickly
recognize the trick and expose the illusion. So, all other things being equal,
the larger the number of people agreeing about the perception, the longer it
lasts and the more modalities or points of view through which it is perceived,
the more real it appears, and the more we can rely on it for making
predictions. More generally, the more independent criteria it fulfills, the
more real it will be.
