We must use it to the extent that it is useful, but there is no need to pronounce judgments of principle about it—e.g., about the limitations of the knowledge obtained with it, its reliability, or the nature of the knowledge determined by it. Engels demonstrated how little the natural construction of the eye determines the boundaries of our knowledge of the phenomena of light. Planck says the same on behalf of contemporary physics. To separate the fundamental psychological concept from the specific sensory perception is psychology’s next task. This sensation itself, self- observation itself, must be explained (like the eye) from the postulates, methods, and universal principles of psychology. It must become one of psychology’s particular problems.

When we accept this, the question of the nature of interpretation, i.e., the indirect method, arises. Usually it is said that history interprets the traces of the past, whereas physics observes the invisible as directly as the eye does by means of its instruments. The instruments are the extended organs of the researcher. After all, the microscope, telescope, telephone etc. make the invisible visible and the subject of immediate experience. Physics does not interpret, but sees.

But this opinion is false. The methodology of the scientific instrument has long since clarified a new role for the instrument which is not always obvious. Even the thermometer may serve as an example of the introduction of a fundamentally new principle into the method of science through the use of an instrument. On the thermometer we read the temperature. It does not strengthen or extend the sensation of heat as the microscope extends the eye; rather, it totally liberates us from sensation when studying heat. One who is unable to sense heat or cold may still use the thermometer, whereas a blind person cannot use a microscope. The use of a thermometer is a perfect model of the indirect method. After all, we do not study what we see (as with the microscope)—the rising of the mercury, the expansion of the alcohol—but we study heat and its changes, which are indicated by the mercury or alcohol. We interpret the indications of the thermometer, we reconstruct the phenomenon under study by its traces, by its influence upon the expansion of a substance. All the instruments Planck speaks of as means to study the invisible are constructed in this way. ‘Tb interpret, consequently, means to re-create a phenomenon from its traces and influences relying upon regularities established before (in the present case—the law of the extension of solids, liquids, and gases during heating). There is no fundamental difference whatsoever between the use of a thermometer on the one hand and interpretation in history, psychology, etc. on the other. The same holds true for any science: it is not dependent upon sensory perception.

Stumpf mentions the blind mathematician Saunderson who wrote a textbook of geometry; Shcherbina (1908)23 relates that his blindness did not prevent him from explaining optics to sighted people. And, indeed, all instruments mentioned by Planck can be adapted for the blind, just like the watches, thermometers, and books for the blind that already exist, so that a blind person might occupy himself with optics as well. It is a matter of technique, not of principle.

Korniov (1922)24 beautifully demonstrated that (1) disagreement about the procedural aspect of the design of experiments makes for conflicts which lead to the formation of different currents in psychology, just as the different philosophies about the chronoscope—which resulted from the question as to in which room this apparatus should be placed during the experiments—determined the question of the whole method and system of psychology and divided Wundt’s school from KUlpe’s; and (2) the experimental method introduced nothing new into psychology. For Wundt it is a correction of self-observation. For Ach~ the data of self-observation can only be checked against other data of self-observation, as if the sensation