All philosophers admit the difference between empirical concepts, such as those of color and refraction, and theoretical ones, such as those of wavelength and refraction index. But not all concur on what to do with the gap in question. Empiricists wish to eliminate all theoretical concepts, or at least to reduce them to empirical concepts. By contrast, realists admit the mutual irreducibility of the concepts of both kinds, and they attempt to relate them, in the way the state of the cardiovascular system of a patient is diagnosed by measuring her blood pressure, and one tries to "read the mind" of a person by observing her behavior.