Psychology cannot attain the certainty and exactness
of the physical sciences, unless it rests on a
foundation of experiment and measurement. A step
in this direction could be made by applying a series
of mental tests and measurements to a large number
of individuals. The results would be of considerable
scientific value in discovering the constancy
of mental processes, their interdependence, and
their variation under different circumstances. Individuals,
besides, would find their tests interesting,
and, perhaps, useful in regard to training, mode of
life or indication of disease. The scientific and
practical value of such tests would be much increased
should a uniform system be adopted, so
that determinations made at different times and
places could be compared and combined. (Cattell,
1890)