 Another claim that Fechner adds to his theory is that the two appearances are connected to one another in a causal or law-like way (see 1861: 211 and ZA: 253). Hence he says explicitly that they are in a relation of interchange (Wechselbedingheit), that they are intimately connected with one another (solidarisch zusammenhangen) (1861: 211). This is notable because some dual aspect theories exclude the possibility of any interaction between the mental and physical appearances precisely because they are such different kinds of attribute. Spinoza, for example, forbade any causal interaction between the mental and physical because they were such different kinds of attribute of substance (Spinoza 1677: Pars Prima, Propositio VI &amp; X). Fechner saw the interaction as problematic, as difficult to explain, but he did not forbid its possibility. Despite all the differences between mental and physical characteristics, he assumed that they referred to distinct events which, somehow, could interact. The whole purpose of his Psychophysik was to explain the interaction between them. 