I can here refer, it will be necessary to present its salient
characteristics along with a description of the special form of
apparatus which was found suitable for use with monkeys and apes.

The method is so planned as to enable the observer to present to any
type or condition of organism which he wishes to study any one or all of
a series of problems ranging from the extremely simple to the complex
and difficultly soluble. All of 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