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Professor Jensen was deeply interested in differential psychology, a field whose central question — What makes people behave and think differently from one another? — strikes at the heart of the age-old nature-nurture debate.

An experiment belongs under what we called "general psychology", and a test under "differential psychology", since the first outcome of a test is to show how the individual differs from others in a certain respect.

Already the outlines of a differential anatomy, and a different physiology and a differential psychology, which will explain to us the unique in the constitution, the temperament and character of an individual, emerge.

What is true of tests in differential psychology is true of the majority of experiments in general psychology: the performer is one person, the observer another, and the observation is objective in character.

In fact, nearly all tests, such as those used in studying differential psychology, are objective.