Kant and the magnitude of sensations

Christian BAUMANN
Justus-Liebig University, Giessen, Germany

Immanuel Kant (1724-1804) published his famous Critique of Pure Reason in 1781. Essentially, this work is a theory of cognition. Kant says that “all human cognition begins with intuitions, goes from there to concepts, and ends with ideas.” He distinguishes three faculties of cognition, i.e., sensibility, understanding, and reason. The Critique concentrates upon pure reason because some of its inferences may give rise to metaphysical illusions.

In that part of his work where Kant describes the “Principles of Pure Understanding”, we find a section on perception. Here, he introduces his doctrine of the real in the appearances (realitas phænomena). Sensations are evoked by the real which has an intensive magnitude, i.e. a degree of influence on sense. Sensations, on the other hand, also have degrees which Kant exemplifies by a brightness estimate (degree of sensation of sunlight expressed as multiples of illuminations from the moon). It seems to have been clear to Kant that both the stimulus (in his text “the real”) and the response (the sensation) can be quantified and that an appropriate mathematics can be applied to their magnitudes. His suggestion to work out this kind of mathematics is found in the Prolegomena to Any Future Metaphysics (§ 24) where he calls it “the second application of mathematics (mathesis intensorum) to natural science.”

Kant’s text referring to the magnitude of sensation is concise and does not always differentiate clearly enough between the stimulus and the resulting response. It nevertheless contains some essential elements of psychophysics as this branch of science was named in 1860 by Gustav Theodor Fechner.

Session VII
Wednesday, 6 July 2005, 1.30 - 2.00 pm

Tenth Annual Meeting of the International Society for the History of the Neurosciences (ISHN) and
Tenth Meeting of the European Club for the History of Neurology (ECHN)

St. Andrews, Scotland, 2005