The claim identifies an assumption that scientific work rests on but cannot itself establish. Investigating nature only makes sense if there is something out there to investigate — something that continues to exist and behave consistently whether or not anyone is watching. Without that, an experiment would record only the experience of the observer.

The word belief is chosen deliberately. The independence of the external world cannot be demonstrated by observation, because every observation reaches us through perception. It is accepted at the outset as the condition that makes measurement, repetition and prediction meaningful.

The position has been contested. Idealist philosophers have argued that talk of a world beyond perception is empty, and some interpretations of quantum mechanics hold that the act of measurement affects what is observed. Einstein resisted the latter view, and the statement reflects his commitment to a reality that exists in its own right.