Brain in a Vat: The Rumour That This World Is a Built Signal

Imagine a laboratory with no windows. In it, a human brain is kept alive in a vat of nutrient fluid. Wires connect to brain areas that create sight, sound, pain, hunger, smell, touch and memory. A computer sends enough signals to make the brain believe it has a body, family, a street to walk on, a phone in its hand and these words in front of it. If every signal matches ordinary life, how could the person in the vat know that they are not in the vat?