state of as being if it is in fact , and so forth. The implication of this is that at time , can predict with certainty that the system is in some eigenstate of , but cannot say which eigenstate it is in, unless itself interacts with the system. An apparent paradox arises when one considers the comparison, between two observers, of the specific outcome of a measurement. In the problem of the observer observed section above, let us imagine that the two experiments want to compare results. It is obvious that if the observer has the full Hamiltonians of both and , he will be able to say with certainty that at time , has a determinate result for 's spin, but he will not be able to say what 's result is without interaction, and hence breaking the unitary evolution of the compound system (because he doesn't know his own Hamiltonian). The distinction between knowing "that" and knowing "what" is a common one in everyday life: everyone knows that the weather will be like something tomorrow, but no-one knows exactly what the weather will be like. But, let us imagine that measures the spin of , and finds it to have spin down (and note that nothing in the analysis above precludes this from happening). What happens if he talks to , and they compare the results of their experiments? , it will be remembered, measured a spin up on the particle. This would appear to be paradoxical: the two observers, surely, will realise that they have disparate results. However, this apparent paradox only arises as a result of the question being framed incorrectly: as long as we presuppose an "absolute" or "true" state of the world, this would, indeed, present an insurmountable obstacle for the relational interpretation. However, in a fully relational context, there is no way in which the problem can even be coherently expressed. The consistency inherent in the quantum formalism, exemplified by the "M-operator" defined above, guarantees that there will be no contradictions between records. The interaction between and whatever he chooses to measure, be it the compound system or and individually, will be a physical interaction, a quantum interaction, and so a complete description of it can only be given by a further observer , who will have a similar "M-operator" guaranteeing coherency, and so on out. In other words, a situation such