 The investigator of psychological phenomena, therefore, must assume, solely for heuristic reasons, two “parallel” and irreducible causal chains by which two distinct types of phenomena may be accounted for (Wundt 1911a: 143; cf. Van Rappard 1979: 109). Wundt compares the distinction between psychological and physiological explanation to the different viewpoints taken by chemistry and physics of the same object, a crystal. The chemical and physical accounts are not of two different entities; rather, they describe and explain the same entity from two distinct points of view, and in this sense the two accounts are “parallel”. Similarly, (neuro-) physiology and psychology do not describe different processes, one neural and one mental, but the same process seen from the outside and the inside, respectively. As Wundt writes,