ABSTRACT
The introduction of psychological concepts into problems of mechanics has its counterpart in the description of the behaviour of living beings in mechanical terms without reference to any motivating or directing element that cannot be directly observed. Some present-day philosophies, indeed, envisage a universe of neutral particulars, events which are in themselves neither material nor mental, but capable of becoming either sense-data or sensations, physical objects or percepts. For the life of every living unit consists of continual effort against the physical forces that threaten it with destruction. It maintains itself only by creating an equilibrium, continually disturbed and continually renewed, between the different physical processes at work in it, so that none of them comes to entire completion. Corresponding to psychological growth, with its continually greater variety and complexity of mental activities, there is, as would be expected, a correlated development of organic complexity.
