First of all, speculation based on Frank's terms in Dune will be guesswork. It is interesting guesswork, and leading us to do it is definitely part of what makes Dune great, but is cannot afford us any certain knowledge about the Dune universe.
Secondly, I just want to point to this book, and this passage:
[quote="Frank Herbert, in "Without Me You're Nothing" p. 33 "]What we have paused to discuss right now is probably the single most important barrier to the widespread useful development of individual computers. It involves a lot of people blathering about their "computer intelligence." According to this scare story, "computer intelligence will win out someday over human intelligence and then we're all going to be in deep trouble. That makes good science fiction drama, but it ain't gonna happen.[/quote]
In other parts of the book (and, incidentally, through the BG and Leto II, he states clearly that he does not believe a computer can be conscious as we are. This is not surprising, giving his knowledge and use of Heidegger. He discusses, IIRC, how computers must think in discrete bits while we have a analog/continuum approach.
The discussion about what he means by good scifi drama will probably erupt soon - I hope in some other thread.
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I have studied the discussions on consciousness in philosophy, and I am not quite convinced one way or the other. Just more confused. But it is a better kind of confusion, I am told...
Anyway, I think the basic question is this: can consciousness and free wil arise from mechanistic principles? If yes, then machines can, in principle, be consciouss too. If no, then you have discarded the idea of a universe that is only matter and its interactions (physicalism).
Heidegger, me and, I think, Frank Herbert believes in the latter.
But that does not mean machines cannot be conscious in a universe that is not 'merely matter'. We could image this or the Dune universe to be one where both humans and machines were imbued with some kind of spirit, or maybe universes where there is really no matter existing in itself (Heidegger and me are still, grossly speaking, onboard for such a universe - I am not going to guess on Frank here...)
Personally, I would not exclude the possibility that machines could be conscious, in such a "there exists only mental states/experiences"-universe. I think Heidegger would, but I am no expert.
Analytical philosophy (not Heidegger!) typically looks at "with what right can we say that X is conscious" and would say that if X behaved as something else we call conscious, then we should ascribe it the same consciousness. So if a machine could act as a human (maybe passing a Turing test would be enough?), we should call it as conscious as we do each other.
Herbert speaks against this possibility both in Dune, and in "Without Me..." I don't know his philosophical leanings, but he strikes me as a Continental guy (siding with Heidegger, a lot of long dead Germans and a bunch of French dudes, basically). But if you are of an analytical bent, then it makes sense that machines in his universe are not conscious - because he does not believe they can act with the same free will and out-of-the-box thinking and existing as we can; so there is no reason to think they are conscious...
As I said, I don't think we can gather anything certain about his choice - he was a lay philosopher and many of the words used are pseudonyms in common use. But if I were to guess, I'd say "sentient" and "conscious" means something like us functionally - that they can think "from scratch", get new ideas, etc. The terminology uses this term about machines, I know - but I don't think he was being precise, especially not when describing anything in a history book, dictionary or encyclopedia.
'Thinking' means analytical - being able to manipulate symbols and draw conclusions. Machines obviously did this, and do this today.