Elements of Psychophysics, Vol. I

...factual functional relationships between the appearance-realms of body and soul as precisely as possible.

What belongs together, quantitatively and qualitatively, distantly or closely, in the world of the body and the world of the mind? According to what laws do their changes follow one another or proceed together? Generally speaking, psychophysics asks these questions and seeks to answer them exactly.

Put another way, though saying the same thing: what belongs together in the inner and outer modes of appearance of things, and what laws exist for their respective changes?

Insofar as a functional relationship exists between body and soul, nothing in itself would prevent us from examining and tracking it in one direction just as much as the other. This can be aptly illustrated by the mathematical functional relationship Functional relationship: A mathematical concept where one variable (like y) changes in a predictable way based on another variable (like x). that exists between the variables x and y of an equation, where each variable can be viewed as a function of the other, and has the other dependent on it for its changes. However, there is a reason for psychophysics to prefer tracking the dependence of the soul upon the body rather than the reverse: it lies in the fact that only the physical is directly accessible to measurement. Meanwhile, the measurement of the mental can only be obtained through its dependence on the physical, as will be shown later. This reason is decisive and determines the direction of our progress in the following pages.

The materialistic reasons for such a preference are neither discussed nor given any weight in psychophysics. The dispute between materialism Materialism: The philosophical belief that everything, including thought and consciousness, results from material interactions. and idealism Idealism: The philosophical belief that reality is fundamentally mental or immaterial.—as it concerns which of the two is the "essence" or cause of the other—remains foreign and indifferent to this field, which relates merely to the world of appearance.

One can distinguish between immediate and mediate In modern terms, we might say "direct and indirect." dependency relationships, or direct and mediated functional connections between body and soul. Sensory sensations stand in immediate dependence on certain activities in our brains, insofar as when one is present, the other is also established, or follows immediately; however, they have only a mediate dependence on those external stimuli which...