The prevailing set or state

It is an aspect of perception in which individual differences as well as differences in the state of an individual at different times, play an important role. It has long been known that particular sets of the individual or attitudes, either long standing or momentary affect the selection of the objects that will be perceived. Phenomenologically they also result in a greater attentive clearness or vividness of those objects. To this aspect of perception the concrete object character of the stimulus is especially relevant, for when we take the character and meaning of the object into account we can often see a relation between it and the state the individual is in. The phenomenon is most clearly shown with respect to objects that we are looking for or meaning that we are seeking to realize from stimulus-situations that are undetermined or vague.

Perceptual sets or readinesses induced by needs are both common and important. Emotional states may also determine perceptual readiness or the manner in which we perceive certain objects or relation. An illusion in which, on a dark night, one sees a tombstone in a cemetery as a ghost is a timeworn example. The way in which indefinite or ambiguous things are perceived may to some extent be influenced by the individual characteristics of the perceiver, a fact implied by the use of the Rorschach test for personality diagnosis. However, overlook the fact that the set to perceive may often be based not upon any strong motivating, emotional o personality edition, but upon nothing more dramatic that the frequency and familiarity of the object in the observers experience.