A concentrated mass of natural, essential will may be compared to a concentated mass of arbitrary, calculative, rational will, in the same way as the organic structure and individual organs of an animal body may be compared to a piece of apparatus or a purpose-built machine. It is easier to study the phenomena being compared if we think of them as visible objects, and an understanding of the contrast between the psychological concepts presented here can be gained from seeing them in this way. Mechanical equipment and biological organs have in common the fact that they contain and represent an accumulation of horse-power or energy which both embodies and increases the total energy of the systems to which they belong. In both cases they possess their own specific strength only in relation to this overall energy and their dependence on it.

They differ, however, in their origins and qualities. An organ is self-generating; additional and more specialised powers are developed to a greater or lesser degree by repeated straining after a particular activity – either of the entire organism or of one of its constituent parts – which the organ in its fully developed state has to achieve. A piece of equipment is made by human hand, which makes use of extraneous material and is given a specific structure and form.