The Problem of Psychological Measurement
Measurement is the breaking up of a quantum of energy into equal units. In undeveloped consciousness we have an original plenum without parts. These are introduced by those processes, generally rhythmical and bilateral, by which the important contacts of touch are mediated through the eye and ear, and which on the biological side are the expression of the means of carrying out the general life-process -- involving the attaining of food, the avoiding of danger and the process of reproduction.
Measurement becomes possible when these rhythmical bilateral processes can be brought into consciousness in tension -- e.g., in estimating a distance. The number of the rhythmical processes required to attain the object constitutes its temporal character, the sum of them its spacial character, the one being expressed in terms of the other. The finer exact measurements of the mathematical sciences are functions of the coarser measurements which the eye can directly effect. The postulate of such finer measurements is a continuum capable of infinite extension and infinite division. The psychological expression of such a continuum is the mediating activity abstracted from the stimulus.
Coming back to the problem of psycho-physical measurement, the quantum measured is, as in the personal equation of the astronomer, a personal error. The least perceptible difference in weight is evidently that limit within which our judgment is unable to detect a difference, and if we depended upon these judgments for our estimate of relative weights it would be an error that the physicist would have to consider. To determine whether this element -- the personal error -- is a psychical one or a physical one we must determine where the line is drawn between the physical and psychical in consciousness. The most important division that we find is between the world as composed of means and of ends. Our objects are teleological, i.e., means to possible ends.
(23) Biologically these means are sensations that act as stimuli to bring us to certain important tactile contacts or to enable us to avoid deleterious ones. This, as has been shown, is the province of measurement. By rhythmical, generally bilateral movements, we break up the continua into equalizable units. We find now that with this process of gaining tactile contacts corresponds the objective spacial world -- a world of teleological objects; that to the world of thought -- the so-called psychical world -- correspond the ends or purposes in consciousness. Measurement can belong, therefore, only to that which is objective and spacial, and the psycho-physical quanta must stand for the physiological elements of our reactions, expressed in personal equations.