prerequisite, and how far psychical substitutions such as a
recognition of the forms and of differences in the light intensity,
may be sufficient for the practical task. Moreover, where not merely
such mental defects, but more subtly shaded variations within normal
limits are involved, it would be superficial, if only the mental
states were examined and not at the same time the mental requirements
of the vocations themselves. The vocation should rather remain the
starting-point. We must at first find out what demands on the mental
system are made by it and we must grade these demands in order to
recognize the more or less important ones, and, especially for the
important ones, we must then seek exact standards with experimental
methods.

Such an experimental investigation may proceed according to either of
two different principles. One way is to take the mental process which
is demanded by the industrial work as an undivided whole. In this case
we have to construct experimental conditions under which this total
activity can be performed in a gradual, measurable way. The psychical
part of the vocational work thus becomes schematized, and is simply
rendered experimentally on a reduced scale. The other way is to
resolve the mental process into its components and to test every
single elementary function in its isolated form. In this latter case
the examination has the advantage of having at its disposal all the
familiar methods of experimental psychology, while in the first case
for every special vocational situation perfectly new experimental
tests must be devised.

Whether the one or the other method is to be preferred must depend
upon the nature of the particular commercial or industrial calling,
and accordingly presupposes a careful analysis of the special