 By 1935 conceptual understanding of the quantum theory was dominated by Niels Bohr’s ideas concerning complementarity. Those ideas centered on observation and measurement in the quantum domain. According to Bohr’s views at that time, observing a quantum object involves an uncontrollable physical interaction with a measuring device that affects both systems. The picture here is of a tiny object banging into a big apparatus. The effect this produces on the measuring instrument is what issues in the measurement “result” which, because it is uncontrollable, can only be predicted statistically. The effect experienced by the quantum object limits what other quantities can be co-measured with precision. According to complementarity when we observe the position of an object, we affect its momentum uncontrollably. Thus we cannot determine both position and momentum precisely. A similar situation arises for the simultaneous determination of energy and time. Thus complementarity involves a doctrine of uncontrollable physical interaction that, according to Bohr, underwrites the Heisenberg uncertainty relations and is also the source of the statistical character of the quantum theory. (See the entries on the  Copenhagen Interpretation  and the  Uncertainty Principle.)