 The notion of direct perception is spelled out by Carnap (1966: ch. 23) by two conditions. Direct perception means, first, perception unaided by technical instruments and, second, that the perception is unaided by inferences. These two conditions are obviously not satisfied for the measurement of quantities like temperature and pressure. For the philosopher, only spatial positions of liquids and pointers are observed when these quantities are measured. To an even higher degree, we are unable to observe electrons, molecules, gravitational forces and genes on this narrow understanding of observability. Hence, expressions referring to such entities qualify as theoretical.