  Fechner founded experimental psychology by developing a method of ‘measuring’ the psychical. Our senses have thresholds of perception; that is, given any amount of stimulation of a sense, there is an amount more of that stimulation which is required for us to notice it. Furthermore, that amount increases logarithmically as the base-line sensation increases. This holds true, with complications and variations, for all of our sensations. The import was that there is a mathematical relationship between the external world and the inner, between the physical and the psychological. It was then possible to talk about these dual aspects of reality in a neutral manner (see Neutral Monism). Fechner uses the word ‘elements’ for this purpose, and Mach’s elements are directly related to them. With some loss, embed Fechnerian methodology within nineteenth century evolutionary theory, and apply it to the categories of physics, and we get Machian epistemology.