Helmholtz Hermann von (1821–94), German physiologist and physicist known for groundbreaking work in physics, physiological optics, perceptual psychology, and the philosophy of geometry. Formally trained as a physician, he distinguished himself in physics in 1848 as a codiscoverer of the law of conservation of energy, and by the end of his life was perhaps the most influential figure in German physical research. Philosophically, his most important influence was on the study of space. Intuitionist psychologists held that the geometrical structure of three-dimensional space was given directly in sensation by innate physiological mechanisms; Helmholtz brought this theory to severe empirical trials and argued, on the contrary, that our knowledge of space consists of inferences from accumulated experience. On the mathematical side, he attacked Kant’s view that Euclidean geometry is the a priori form of outer intuition by showing that it is possible to have visual experience of non-Euclidean space (‘On the Origins and Meaning of Geometrical Axioms,’ 1870). His crucial insight was that empirical geometry depends on physical assumptions about the behavior of measuring instruments. This inspired the view of Poincaré and logical empiricism that the empirical content of geometry is fixed by physical definitions, and made possible Einstein’s use of non-Euclidean geometry in physics. See also PHILOSOPHY OF MATHEMAT- ICS , POINCARÉ . R.D.