Phenomenological theories—like thermodynamics and S-R psychology—are often commended on account of their acknowledged generality, and at other times because of their alleged philosophical virtue of not trespassing on the description of phenomena, by abstaining from introducing dubious occult entities such as atoms or will. Both scientific theories and their referents have often been likened to devices in the form of boxes with external dials that can be manipulated. The dials correspond to the "external" variables representing observable properties, such as the size and direction of motion of visible bodies; the pieces inside the box correspond to the "internal" or hypothetical variables, such as elastic strain and atomic weight. Black box theories are, then, those whose variables are all external and global, whether directly observable (like the form and color of perceptible bodies) or indirectly measurable (like temperature and potential difference). The terms 'black box,' 'external,' and 'nonrepresentational,' which are all equivalent, seem preferable to 'phenomenological,' a highly equivocal qualifier.