 According to Whewell, all knowledge has both an ideal, or subjective dimension, as well as an objective dimension. He called this the “fundamental antithesis” of knowledge. Whewell explained that “in every act of knowledge … there are two opposite elements, which we may call Ideas and Perceptions” (1860a, 307). He criticized Kant and the German Idealists for their exclusive focus on the ideal or subjective element, and Locke and the “Sensationalist School” for their exclusive focus on the empirical, objective element. Like Francis Bacon, Whewell claimed to be seeking a “middle way” between pure rationalism and ultra-empiricism. Whewell believed that gaining knowledge requires attention to both ideal and empirical elements, to ideas as well as sensations. These ideas, which he called “Fundamental Ideas,” are “supplied by the mind itself”—they are not (as Mill and Herschel protested) merely received from our observations of the world. Whewell explained that the Fundamental Ideas are “not a consequence of experience, but a result of the particular constitution and activity of the mind, which is independent of all experience in its origin, though constantly combined with experience in its exercise” (1858a, I, 91). Consequently, the mind is an active participant in our attempts to gain knowledge of the world, not merely a passive recipient of sense data. Ideas such as Space, Time, Cause, and Resemblance provide a structure or form for the multitude of sensations we experience. The Ideas provide a structure by expressing the general relations that exist between our sensations (1847, I, 25). Thus, the Idea of Space allows us to apprehend objects as having form, magnitude, and position. Whewell held, then, that observation is “idea-laden;” all observation, he noted, involves “unconscious inference” using the Fundamental Ideas (see 1858a, I, 46). Each science has a Particular Fundamental idea which is needed to organize the facts with which that science is concerned; thus, Space is the Fundamental Idea of geometry, Cause the Fundamental Idea of mechanics, and Substance the Fundamental Idea of chemistry. Moreover, Whewell explained that each Fundamental Idea has certain “conceptions” included within it; these conceptions are “special modifications” of the Idea applied to particular types of circumstances (1858b, 187). For example, the conception of force is a modification of the Idea of Cause, applied to the particular case of motion (see 1858a, I, 184–5 and 236).