1. A method for measuring perception of an observer, the method using a stimulus output device for presenting stimuli to the observer and a stimulus manipulation device permitting the observer to modify the presented stimuli by selecting related stimuli from a database of stimuli, the method comprising the steps of: a) selecting a first stimulus, a second stimulus, and a third stimulus from the database of stimuli, said selected stimuli represented as stimulus points contained in a stimulus space, said first, second, and third stimuli presented to the observer by the stimulus output device in sequential order; b) determining an observer-defined fourth stimulus such that the observer perceives the fourth stimulus to be related to the third stimulus in the same way as the observer perceives the second stimulus to be related to the first stimulus; c) selecting a new first, a new second, and a new third stimulus from the database of stimuli, said new stimuli being represented as stimulus points contained in the stimulus space, at least one of said new stimuli corresponding to a stimulus point that is different from the stimulus points corresponding to the previously selected first, second, and third stimulus; d) determining a new observer-defined fourth stimulus such that the observer perceives the new fourth stimulus to be related to the new third stimulus in the same way as the observer perceives the new second stimulus to be related to the new first stimulus; and e) performing steps (c) through (d) until a predetermined number of sets of four stimuli are determined such that the observer perceives the second stimulus of each set to be related to the first stimulus of each set in the same way as the observer perceives the fourth stimulus of each set to be related to the third stimulus of each set.