Thought experiment: the brain is closed in the dish and stimulated by an apparatus connected to receive stimuli. This apparatus (or scientist through it) creates a perfectly coherent illusion of the existence of persons, objects of everyday experience (however, all experiences are actually the result of electrical impulses sent by the computer). You can go ahead and assume that all people (all sensory organisms) are the brains (nervous systems) in the vessels connected to the system that generates a collective hallucination.
Hilary Putnam