Direct psychophysical scaling procedures presuppose that observers are able to directly relate a numerical value to the sensation magnitude experienced. This assumption is based on fundamental conditions (specified by Luce, 2002), which were evaluated experimentally. The participants' task was to adjust the loudness of a 1-kHz tone so that it reached a certain prespecified fraction of the loudness of a reference tone. The results of the first experiment suggest that the listeners were indeed able to make adjustments on a ratio scale level. It was not possible, however, to interpret the nominal fractions used in the task as "true" scientific numbers. Thus, Stevens's (1956, 1975) fundamental assumption that an observer can directly assess the sensation magnitude a stimulus elicits did not hold. In the second experiment, the possibility of establishing a specific, strictly increasing transformation function that related the overt numerals to the latent mathematical numbers was investigated. The results indicate that this was not possible for the majority of the 7 participants.