 When we consider elements like red, green, hot, cold and the rest, which are physical and mental in virtue of their dependence on both external and internal circumstances, and are in both respects immediately given and identical, the question as to illusion and reality loses its sense. Here we are simultaneously confronted by the elements of the real world and of the ego. The only possible further question of interest concerns their functional interdependence…. (Mach 1905: 7–8)