Why are people so skeptical about the notion that a sufficiently advanced

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Why are people so skeptical about the notion that a sufficiently advanced computer program could replicate human intelligence (meaning free will insofar as humans have it; motivation and creativity; comparable problem-solving and communicative capacities; etc.)? If humans are intelligent in the way we are because of the way our brains are built, than a computer could be constructed that replicates the structure of our brains (incorporating fuzzy logic, neural networks, chemical analogs, etc). Worst comes to absolute worst, a sufficiently powerful molecular simulator could run a full simulation of a human brain or human body, down to each individual atom. So there doesn't seem to be anything inherent in the physicality of humans that makes it impossible to build machines with our intelligence, since we can replicate physical structures in machines easily enough. If, however, humans are intelligent for reasons that do not have anything to do with the physical structure of our brains or bodies - if there is some immaterial reason for consciousness, free will or other aspects of our intelligence - than we're essentially talking about souls. And if souls don't just supervene on physical phenomena (which is the entire nature of this fork of the problem - if they did supervene, we'd be back at the first point), then why shouldn't machines, too, be able have souls? Maybe they already do. The only way to escape this and continue to assert that machines could never possess human intelligence is to say that there is a god, or a group of gods, who decide what gets to have souls and what doesn't, and machines aren't on the list. But outside of theistic circles, this argument can't be expected to carry any weight for as long as people are skeptical about theism in general. So what leads so many people to believe that machines could never replicate a human intelligence?