"[Herbert Spencer's (e.g. 1855, The Principles of Psychology) view of the ontogeny, or individual development, of intelligence in humans, from birth to maturity, is that it has three main aspects: (a) an increase in the accuracy of inner adjustments to outer demands, (b) an increase in the number of items of simple knowledge, and (c) an increase in the number of items of consciousness of the external environment. The idea of accuracy of perceptions was likely a precursor of Francis Galton's (1912-1911) emphasis on sensory discrimination as a measure of intelligence...."