If our interaction with our world is solely the result of billions to trillions of electro-chemical reactions and patterns, then a "false" reality being "fed" into our brain would still be true and completely valid. If you took one person sitting in a room and a second person having their synaptic patterns being forced to mimic that of a person sitting in the same room, both person's perceived realities would be entirely identical and thus entirely valid and true. I like to think of it mathematically. If, given a, b is equal to c (in other words, b(a) = c), then a second equation, with the exact same assumptions and constraints as the first (the second equation is in the same "reality" of the first), if b is determined to equate to c, then the input to that equation must be a. Our realities are the manifestations of our senses of our environment. If a theoretical being is said to truly exist in the same reality as we perceive ourselves to be, it would have the same neural impulses as we do (removing fluctuations for personal experiences and all that). If that theoretical consciousness has no additional data, factors, or attributes that make it's assuredly true perceptions of it's reality more true, than our reality is equally as valid as the reality experienced by a truly existing being, meaning we are just as extant as can be.