Bohr was clear that:
The entire formalism is to be regarded as a tool for deriving predictions, of definite or statistical character, as regards information obtainable under experimental conditions described in classical terms.
We can't know anything about the Universe without doing experiments: making "observations". Under [the Copenhagen Interpretation], as described above, there is a line of demarcation somewhere between the quantum objects being observed, and the "classical" world perceived by the observers. This line is sometimes referred to as the "Heisenberg split".
Now, Bohr was also very clear that the instruments themselves are on the classical side of the divide:
In actual experimental arrangements, the fulfillment of such requirements is secured by the use, as measuring instruments, of rigid bodies sufficiently heavy to allow a completely classical account of their relative positions and velocities.
Others, including von Neumann, have reasoned in various ways to move Heisenberg's split all the way to the level of consciousness: under that
interpretation of the Copenhagen Interpretation, the wavefunction of the radioactive source, Geiger counter, poison and cat is only ever "collapsed" when perceived by a conscious observer.
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Originally Posted by grant hutchison
