f the problems, however, are completely
soluble by an organism of excellent ideational ability. For the human
subject, the solution of the easiest problem of all requires almost no
effort, whereas even moderately difficult problems may require many
repetitions of effort and hours or days of application to the task. In
each case, the solution of the problem depends upon the perception of a
certain constant relation among a series of objects to which the subject
is required to attend and respond. Such relations are, for example,
secondness from one end of the group, middleness, simple alternation of
ends, or progressive movement by constant steps from one end of a group
to the other.

It is possible to present such relational problems by means of
relatively simple reaction-mechanisms. In their essential features, all
of the several types of multiple-choice apparatus designed by the writer
and used either by him or by his students and assistants are the same.
They consist of a series of precisely similar reaction-devices, any one
or all of which may be used in connection with a given observation.
These reaction-mechanisms are so chosen as to be suited to the structure
and action-system of the animal to be studied. For the human being the
mechanism consists of a simple key and the total apparatus is a bank of
keys, with such electrical connections as are necessary to enable the
observer to obtain satisfactory records of the subject's behavior. Let
us suppose the bank of keys, as was actually the case in my first form
of apparatus, to consist of twelve separate reaction-mechanisms; and let
us suppose, further, the constant relation (problem) on the basis of
which the subject is required to react to be that of middleness. It is
evident that in successive trials or experiments the keys must be
presented to the subject in odd groups, the possibilities being groups
of 3, 5, 7, 9, or 11. If for a particular observation the experimenter
wishes to present the first three keys at the left end of the keyboard,
he pushes back the remaining nine keys so that they cannot be operated
and requires the subject to select from the group of three keys the one
which on being pressed causes a signal to appear. It is of course the
clearly understood task of the subject to learn to select the correct
key in the group on first trial. This becomes possible only as the
subject observes the relation of the key which produces the desired
effect to the