The present paper is concerned with Helmholtz’s theory of measurement. It will be argued that an adequate understanding of this theory depends on how Helmholtz’s application of the concepts of perception and coincidence is interpreted. In contrast both to conventionalist and Kantian readings of Helmholtz’s theory, a more realistic interpretation will be suggested.

This article considers Helmholtz’s relation to philosophy, including Fichte’s philosophy. Recent interpreters find Fichtean influence on Helmholtz, especially concerning the role of voluntary movement in distinguishing subject from object, or “I” from “not-I.” After examining Helmholtz’s statements about Fichte, the article describes Fichte’s ego-doctrine and asks whether Helmholtz could accept it into his sensory psychology. He could not accept Fichte’s core position, that an intrinsically active I intellectually intuits its own activity and posits the not-I as limiting and determining that (...) activity, because that account requires cognitive abilities that conflict with Helmholtz’s claim that the development of spatial perception relies primarily on associative memory and involves no direct awareness of the ego’s activity as such. Helmholtz would have known various accounts of the distinction between subject and object, or self and world, from sensory physiology, including associative accounts invoking voluntary motor movements and accounts describing checks on the perceiver’s activity. Some of the accounts cite Fichte on the I and not-I, but most do not, and none adopt Fichte’s Idealism. Finally, the article examines Helmholtz’s relation to metaphysics, finding that in all periods he rejected Fichtean Idealism and that the modest metaphysics of his mature period is a version of structural realism. (shrink)