The computer would then be simulating reality (including appropriate responses to the brain's own output) and the "disembodied" brain would continue to have perfectly normal conscious experiences, such as those of a person with an embodied brain, without these being related to objects or events in the real world.
Since the brain in a vat gives and receives exactly the same impulses as it would if it were in a skull, and since these are its only way of interacting with its environment, then it is not possible to tell, from the perspective of that brain, whether it is in a human body or a vat. Yet in the first case most of the person's beliefs may be true (if they believe, say, that they are walking down the street, or eating ice-cream); in the latter case their beliefs are false.
Since the argument says one cannot know whether one is a brain in a vat, then one cannot know whether most of one's beliefs might be completely false. Since, in principle, it is impossible to rule out oneself being a brain in a vat, there cannot be good grounds for believing any of the things one believes; a skeptical argument would contend that one certainly cannot know them, raising issues with the definition of knowledge.
The brain in a vat is a contemporary version of the argument given in Hindu Maya illusion.
Read more at https://en.wikipedia.org/wiki/Brain_in_a_vat