egal aspect of the problem,[53] first turned
my attention to the psychological difficulty involved.

When I approached the question in the Harvard psychological
laboratory, it was clear to me that the degree of attention and
carefulness which the court may presuppose on the part of the customer
can never be determined by the psychologist and his experimental
methods. It would be meaningless, if we tried to discover by
experiments a particular degree of similarity which every one ought to
recognize or a particular degree of attention which would be
sufficient for protection against fraud. Such degrees must always
remain dependent upon arbitrary decision. They are not settled by
natural conditions, but are entirely dependent upon social agreement.
A decision outside of the realm of psychology must fix upon a
particular degree in the scale of various similarity values as the
limit which is not to be passed. The aim of the psychologist can be
only to construct such a scale by which decisions may be made
comparable and by which standards may become possible. The experiment
cannot deduce from the study of mental phenomena what degrees of
similarity ought to be still admissible, but it may be able to develop
methods by which different degrees of similarity can be discriminated
and by which a certain similarity value once selected can always be
found again with objective certainty. After many fruitless efforts I
settled on the following form of experiments, which I hope may bring
us nearer to the attainment of the purpose.

A group of objects is observed for a definite time and after a
definite interval another group of objects is offered for comparison.
This second group is identical with the first in all but one of the
objects, and this is replaced by a similar one. The question is how
often this substitution will be noticed by the observers. I may give
in detail a characterization of the set of experiments in which we are
at present engaged. We are working with picture postal cards, using
many hundred cards of different kinds, but for each one we have one
or several similar cards. As postal cards are generally manufactured
in sets, it is not difficult to purchase pairs of pictures with any
degree of similarity. Two cards with Christmas trees, or two with
Easter eggs, or two with football players, or two with forest
landscapes, and so on, may differ all the way from a slight variation
of color or a hardly noticeable change