The question of validity is especially important whenever an operational definition is used to measure intelligence? The answer is, we cannot. Intelligence is hypothetical and cannot be directly observed or measured. The best we can do is to measure intelligent behavior or some other external manifestation of intelligence. In the past, researchers have attempted to measure intelligence by measuring brain size (bigger brain equals greater intelligence) and bumps on the skull. Operationally, defining intelligence in terms of brain size or bumps probably seems silly, but at one time, these were viewed as valid measures of intelligence