No no, the burden is on the person who claims something "impossible." You have no basis to claim that what he said is impossible.

Well I'm not saying I know for sure, just that in some senses we tend to view algorithms as living in an abstract world of mathematics, but the reality is that an actual computation is a physical occurrence in universe. So what we might end up with is an intelligent chunk of the universe being inside of a computer and not a human, and that might not tell us as much as we'd hoped because the intelligence is an emergent property of the computation and not a defined behavior of the algorithm.

Any simulation running neurone interactions has the ability to produce sentience without us knowing why or how it happened. However it would require a phenominal amount of processing power than the simulated 'brain' would have. Human neurones can have up to 10000 synapses connecting them together (on average 7000 per neurone), which each synaptic interaction is a multi-variable of chemical reactions, catalysts, reinforcers, inhibitors and stimulants.

I think our aim should be to simulate the R-Complex part of the triune brain model. This is where our evolutionary intelligence came from and it is the best basis to start. Birds have managed high levels of intelligence without the neocortex present in mammals, so obviously there's more than one route to sentience.

I believe brain density is the key to sentience. In a small animals, like birds, they lose heat quickly and efficiently, which means their 'processors' can pump out more heat without burning out. Where as a human brain would cook itself if it was 'overclocked' as we're at the upper limit of the communication-efficiency ratio, and Elephants are above that ratio but get away with large processing and data storage with a form of 'low-energy' brains.

With the speed of electricity in synapses, a Crow might be able to achive 10 times as many messages in the time it takes one of our brains to make 1 message. Elephants might only make .5 messages a second per synapse. So a crow would benefit greatly by increasing brain density and be able to solve problems to the same degree as a human infant, however humans have phenominal storage in our brains and a crow simply doesn't have the brain mass to compete so we have more data to act on than a crow. Elephants use their brains to remember pretty much everything, they've even been seen taking short cuts that they have 'guessed' between paths they've taken before.

I think the brain is astounding, but to reproduce sentience we should follow evolution; not only because it gives us vast insights into how our brains evolved, but also because it should avoid us ever getting into the irrational AI situation. If an AI is made from simulating a human brain then it is human, which it would be capable of understanding as it would have to be 'taught' and I know I'm human because of the society I know around me that teaches me that I think and act in a similar method as everyone else, just with different data.

:Well, that should teach you for asking why.