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 economical processes. It is, indeed, easy to recognize that in certain industrial activities a series of psychical functions is in question which all lie side by side and which do not fuse into one united total process, however much they may influence one another. But for many industrial tasks just this unity is the essential condition. The testing of the mental elements would be in such cases as insufficient as if we were to test a machine with reference to its parts only and not with reference to its total united performance. Even in this latter case this unified function does not represent the total personality: it is always merely a segment of the whole mental life. We may examine with psychological methods, for instance, the fitness of an employee for a technical vocation and may test the particular complex unified combination of attention, imagination and intelligence, will and memory, which is essential for that special kind of labor. We may be able to reconstruct the conditions so completely that we would feel justified in predicting whether the individual can fulfill that technical task or not; and yet we may disregard entirely the question whether that man is honest or dishonest, whether he is pacific or quarrelsome; in short, whether his mental disposition makes him a desirable member of that industrial concern under other aspects.