AbstractThe philosophical convictions of Hermann von Helmholtz and the empiricist psychology he developed have been extensively discussed in historical literature. This literature has not usually emphasized the tacit assumptions about human physiology that underlaid these convictions nor the way in which Helmholtz's epistemology served as a methodological directive in his research. Helmholtz assumed nerve transmission between sense organs and the mind to be a passive process. Distortion in stimulus patterns occurs physically in the sense organs, which can therefore be treated through mechanical analogies. Stimuli become converted to the perceptions of consciousness through mental processes that are essentially analogous to conscious, inductive inference and that are therefore susceptible, in principle, to introspective investigation. This view of mental function reflected Helmholtz's intellectual debt to German idealism, especially to the philosophical views of J. G. Fichte.