Well we just make sure its not garbage in, that is all. The human brain is only a finite object, if we simulate enough of it, we reproduce a human personality.
1) How much is "enough of it" and how do you collect that data without altering it in the process?
2) Even in that case it isn't a reproduction as much as a simulation. Remove the stored variables, the simulation immediately ends.
You don't need subatomic precision to simulate a human being, you need only cellular precision
Except that human beings are subject to all kinds of microscopic influences, particularly genetic, hormonal, biochemical and neurological, all of which involve chemical interactions at the molecular, sub-molecular and -- yes -- subatomic scale.
Although, I agree that the whole point of a simulation is to APPROXIMATE something with a reasonable amount of fidelity, usually with the intention of reproducing the behavior of a thing without actually having the thing itself. In which case, the simulated behavior of, say, a non-player character in Call of Duty 3 is no different from a robot running a simulation of a human brain. The only difference is realism.
One can for instance predict the weather without simulating the path of every molecule and atom in our atmosphere
And the computer models used to predict weather patterns have huge margins for error as a result. For similar reasons, there is not now nor will there ever be a computer model capable of making accurate predictions for turbulent-flow fluid dynamics.
The human brain is such a complex organ that an error margin of around 30% can cause its basic processes to collapse altogether. It's not even that it would cause the brain to behave in ways uncharacteristic of a human, it would cease to function effectively in any way shape or form and exhibit something not totally unlike debilitating epilepsy.
Telepresence is when an actual real human being operates a robot by remote control, if that robot is simulating a human being internally, then its not real telepresence as its not remotely operated but internally operated.
If it's not expressing the actions of a real person, then it's not expressing live human consciousness, not even vicariously. It is -- in a word -- SIMULATING human consciousness. The purpose of a simulation is to reproduce the behavior of a thing without actually reproducing the thing itself. In other words, a thing that acts like a person without actually BEING a person.
Put the Blade Runner scenario aside and think of the actual MOVIE. Harrison Ford is an actor; as such, a few scenes at a time, he is SIMULATING an individual named Rick Deckard. Like most actors, he does this by internalizing -- or otherwise inventing -- characteristics about Deckards background, his motivations, his emotional states, his fears, his desires, his pain threshhold, etc. He is simulating a completely different person who otherwise does not really exist, and because this is Harrison Ford we're talking about, he's doing a damn good job of it.
But no matter how good an actor plays the role, the role remains fictional. Harrison Ford does not forget who he is and BECOME Rich Deckard, nor would he even if he was forced to play this role all day every day for years at a time. Only the BEHAVIOR has any realism to it; the character and personality of Rick Deckard does not.
To complete this analogy: take an android with some stupendous personality simulation software, sublimely designed so that you can feed it some information about a specific person and it will behave very close to the way that person would behave in real life. You program this android with Rick Deckard's fictional life story, details about his career, his love life, his emotional states, his future aspirations, even some supplemental details about the world he lives in for context. The android extrapolates this data and produces behavior accordingly. Meanwhile, your EEG is picking up a staggering amount of electrical activity in the android's CPU as it works through its scenes and produces the needed behavior.
Is that electrical activity the mind of Rick Deckard? No it is not. Because "everything we know about Rick Deckard" has been reduced to an equation, the android's brain is the calculator, and its behavior is the output of the equation.