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There is a long-standing debate about whether gravity should be quantised. Apowerful line of argument in favour of quantum gravity considers models ofhybrid systems consisting of coupled quantum-classical sectors. The conclusionis that such models are inconsistent: either the quantum sector's definingproperties necessarily spread to the classical sector, or they are violated.These arguments have a long history, starting with the debates about thequantum nature of the electromagnetic fields in the early days of quantumtheory. Yet, they have limited scope because they rely on particular dynamicalmodels obeying restrictive conditions, such as unitarity. In this paper wepropose a radically new, more general argument, relying on less restrictiveassumptions. The key feature is an information-theoretic characterisation ofboth sectors, including their interaction, via constraints on copyingoperations. These operations are necessary for the existence of observables inany physical theory, because they constitute the most general representation ofmeasurement interactions. Remarkably, our argument is formulated withoutresorting to particular dynamical models, thus being applicable to any hybridsystem, even those ruled by "post-quantum" theories. Its conclusion is alsocompatible with partially quantum systems, such as those that exhibit featureslike complementarity, but may lack others, such as entanglement. As an example,we consider a hybrid system of qubits and rebits. Surprisingly, despite therebit's lack of complex amplitudes, the signature quantum protocols such asteleportation are still possible.

Why we need to quantise everything, including gravity