This is a recycled version of the old “How do you know you’re not a brain in a jar being kept alive by a mad scientist who is stimulating you to think you are perceiving the world?” One fallacy here is that the person raising the question wants to cast doubt on all human knowledge while relying on the fact you both know what are brains, jars, and mad scientists. Another fallacy is the fact that the question involves an assertion (that you might be just a brain in a jar) with no evidence. You need evidence to know and evidence to doubt. Arbitrary assertions without evidence are not worth listening to.
John Zakour and Scott Roberts