It seems that one day he awoke, having dreamed he was a butterfly. After a moment's reflection, he wondered if he were instead a butterfly, dreaming that he was a man. As we saw in the brains in vats scenario, all we experience is the consequence of a stream of nerve impulses. The conventional picture of a real, external world is not the only possible explanation for what our nervous system is reporting to us. Rene Descartes realized that it is possible that there is an evil mad scientist experimenting on our disembodied brains.
Scientists place great faith in the evidence of our senses.
All human beings, do. Most people are skeptical about flying saucers, not because the notion of visitors from other worlds is inherently unreasonable, but simply because no one has yet to produce unquestionable sensory evidence for their existence. Fuzzy photographs and incredible tales by eyewitnesses simply are not enough, for most of us, because we can think of other, more pedestrian explanations for the photographs and they stories of the eyewitnesses. However, the postulation of the brains in vats scenario turns such thoughts inside out. There simply is no empirical evidence possible to disprove it.