 Natural science has a constructive character. The phenomena with which it deals are not independent manifestations or qualities which can be read off from nature, but can only be determined by means of an indirect method, through interaction with other bodies. Their implicit definition is bound up with definite natural laws which underlie the interactions. Consider, for example, the introduction of the Galilean concept of mass which essentially comes down to the following indirect definition: “Each body possesses a momentum, that is, a vector \(m\overline{v}\) which has the same direction as its velocity \(\overline{v}\)—the scalar factor \(m\) is called its mass. The law of momentum holds, according to which the sum of the momenta before a reaction between several bodies is the same as the sum of their momenta after the reaction.” By applying this law to the observed collision phenomena, one obtains data for the determination of the relative masses. The scientific consensus was, however, that such constructive phenomena can nevertheless be attributed to the things themselves even if the manipulations, which alone can lead to their recognition, are not being carried out. In Quantum Theory we encounter a fundamental limitation to this epistemological position of the constructive natural science.