The truth condition for "This is one mind" is roughly that this consists in a single realisation of the mind blue-print. The ideal way of establishing its truth would be to investigate the structure of what confronts us and determine whether it realises the mind blue-print. Unfortunately, we are not sufficiently knowledgeable. If we did not know that brains animated the people we see around us, there would be no temptation to suppose that a brain in a vat was a mind. The statement has to be verified indirectly.