Since the maxim is initially delivered by a detective who is trying to solve human crimes in a realistic universe, I think we can safely assume that by "impossible", he did not mean to descend into solipsism.
Otherwise every locked-room mystery could be solved by "the killer traveled through a folded pocket universe", or "the killer had telekinetic powers and managed to stab the victim in the back from outside the room". And similarly, seances and faeries are improbable enough to be grouped with the impossible for now.
Otherwise every locked-room mystery could be solved by "the killer traveled through a folded pocket universe", or "the killer had telekinetic powers and managed to stab the victim in the back from outside the room". And similarly, seances and faeries are improbable enough to be grouped with the impossible for now.