Since at least the time of Francis Bacon, 400 years ago, scientists have recognized that they face two tasks. They must describe the phenomena in their purview, and they must explain the phenomena they observe. When stated this way, science seems straightforward. First one collects and systematizes a large number of descriptions and then one induces explanatory principles.

If only things were so easy. The tasks of description and explanation generally cannot be disjoined and taken up one at a time. Description and explanation go hand in hand. It is hard to determine what constitutes a meaningful description without some sense of explanatory principles. As Russell (1945) explained, in commenting on Bacon, “usually some hypothesis is a necessary preliminary to the collection of facts, since the selection of facts demands some way of determining relevance. Without something of this kind, the mere multiplicity of facts is baffling” (p. 545).

The standard difficulties are compounded in personality psychology. Throughout most of its history, the field has harbored multiple theories. Different theories admit different types of description as meaningful. The question of whether to describe personality through objective tests, projective tests, narratives, behavioral observations, or physiological recordings has divided personality psychologists over the years (see, e.g., Craik, 1986).

The intermingling of description and explanation, however, cuts far deeper than merely to the question of measurement technique. Different theories implicitly pursue different types of explanation.