The present contribution discusses aspects of exo- and endo-descriptions which arise in psychophysical investigations of perception processes. An approach connecting physics and perceptions has long traditions. In fact, some central foundations of modern science were laid when people began to think about the relations between internal and external qualities, e.g., by connecting the sensation of tones with the variation of the external system parameters, such as the lengths of flutes or monochords. The first decisive bifurcation from qualitative to quantitative science was the finding of the Pythagorean school, that mathematics can account for certain connections of the external and internal world, as it is the case in perceiving consonant intervals.