In his explanation of color vision, Hering postulated red-green, yellow-blue, and black-white receptors on the retina that could either be torn down (causing the color experiences of red, yellow, and white, respectively), or built up (causing the experiences of green, blue, and black, respectively). Wundt thought it was necessary to write his Volkerpsychologie b/c According to him, the nature of the higher mental processes could be deduced from the study of such cultural products as religion, social customs, myths, history, language, morals, art, and the law. Wundt studied these topics for the last 20 years of his life, with his research culminating in his 10- volume Völkerpsychologie ("folk"or"cultural" psychology). In this work, Wundt emphasized the study of language through historical analysis & naturalistic observation. The decline of structuralism can be seen for many reasons for instance, the unreliability of introspection because it yielded different results, lack of agreement on descriptions that the stimulus displayed, as well as it being more retrospection that introspection b/c the events reported had already occurred, also that one could not introspect on something w/ out that very observation changing it. Furthermore, structuralism ignored the psychological development, abnormal behavior, personality, learning, individual differences, evolutionary theory, and practicality which also led to its downfall.