You don't. But it's solipsism to think otherwise, and while solipsism is hard to argue against logically it's not a very interesting or useful way of navigating the world we experience. We can't prove other people aren't p-zombies but the value bet is definitely that, appearing like us in every other way, they also experience like us.
It doesn't need to be solipsism - for instance, maybe half of us are conscious.
But if we can't even know that, if we don't even have a test to see whether some human or animal is conscious or not, how can we start trying to figure out what makes them conscious? It seems it's impossible to get to something falsifiable without such a test.
Like, you say a rock isn't conscious. But what about a sponge? An amoeba? How can you answer that if you can only guess answer whether your neighbour is?
It is very relevant to keep analyzing and keep trying to get any other answer to this question, because while "appearing like us in every other way, they also experience like us" applies to other humans, as soon as we want to talk about the consciousness (or lack of it) of other actors, this argument can not be applied and we would very much like to get to any other criteria of consciousness which could be applicable to arbitrary non-human agents.
Even if we axiomatically assume that everyone else is not a p-zombie, trying to find any evidence towards your/mine consciousness other than that axiom is helpful as a candidate for such criteria which can be tested and validated.
All the people I love the most aren't actually real, only I am – if one seriously believes that, it is going to do a great deal of harm to one's mental health.
Solipsism can ethically justify all kinds of horrors. "Other people only exist in my own mind, so if I murder/torture/etc them, those acts are just figments of my own imagination: there is little ethical difference between murdering someone for real and watching a murder on TV"
If a belief is impossible for a human being to seriously believe while maintaining their health, sanity and humanity, I think that in itself is a good argument that the belief must be false.
Why? Why can't the world be a cruel and indifferent place? Take for example the babies that had to be left behind by hospital staff in one of the Gaza hospitals when it was occupied by the IOF and when the doctors could come back a few weeks later they found the rotting corpses of these babies who had been left to starve, alone and afraid, by the Israeli soldiers.
If you were one of these newborns and somehow con-cious and you had to choose between 'I have been left here to die' and 'Mommy loves me and is coming soon', would you reject the former as obviously false since it's incompatible with health, sanity, and humanity?
I think so easily dismissing the cruelty and insanity of the world is in itself inhumane.
> Why? Why can't the world be a cruel and indifferent place?
Society runs on faith–that the cruelty and insanity of the world, while undeniable, has its limits. Historically (and even for the majority of the global population today), that faith was most often religious, but it also comes in secular versions – everyone from communists to LGBT activists to the New Atheist movement has a faith that history is "on their side", even if they do not believe in any divine assurance of that. A society in which everyone (or even the clear majority) have given up faith and hope, is a society doomed to wither and die, and be replaced by societies which still retain those things (if there be any other societies retaining that faith left to replace it).
The problem with solipsism, is not that it supposes the world is sometimes cruel and insane, but that it destroys one's faith that said cruelty and insanity has any limits. And without that faith, the continued functioning of society becomes impossible.
Does that have any relevance to the tragic case of a newborn abandoned to starve? They can't constitute a society, so concerns of what beliefs are necessary for society to function aren't relevant to them.
> If you were one of these newborns and somehow con-cious and you had to choose between 'I have been left here to die' and 'Mommy loves me and is coming soon', would you reject the former as obviously false since it's incompatible with health, sanity, and humanity?
If believing that "Mommy loves me and is coming soon" gives comfort to a dying child, and eases (however slightly) the pain of their horrific death, then I would want them to believe it–and if I were them, I would want to believe it too. It is better for a dying child to believe comforting falsehoods than painful truths–truth has no value for them, and falsehoods can do them no harm.
> We can't prove other people aren't p-zombies but the value bet is definitely that, appearing like us in every other way, they also experience like us.
Logic doesn't need to be binary. There is no need for the answer to such a question to even be defined.
Solipsism is incoherent because it's not radical skepticism. All of the critique of the external world also apply to belief in the primacy on internal experience. Any good solipsist should just accept the "evil demon" of descartes, embrace radical doubt, and say "I don't even know if I truly exist or not".
"I don't know if I'm a P zombie, and I don't know if I'm a replicant or not, Deckard!"
well doesn't the argument suggest the only thing you can be certain of is I, or at least some 'experiencing agent' exist, otherwise there would be no subject to do the experiencing
Yeah the first-person subjectivity has to arise before second and third persons can arise. But with some further investigation, one can find that the things they take to be their subject are in fact object to them, too.
