The ultimate signature of sentience has nothing to do with mimicking humanity, nor with demonstrating the ability to set goals and make plans.

The real mystery of sentience is the ability to witness. It's the ability to notice when you feel good or bad, or warm or cold, or any other of the vast spectrum of experiences we witness via our senses.

Unfortunately, we don't have a way of detecting this directly. The only sentience we actually know of beyond all doubt is our own. This is why solipsism is a thing.

But most of us are willing to draw an analogy between ourselves and other people, and make a small logical jump to assume that other humans also witness things - and are therefore sentient - like us. And we make a similar jump to assume that our favorite animals also witness things.

But it's hard to know where to stop. Because if dogs and cats are sentient, then presumably, so too, are birds and lizards. And potentially smaller things like bugs. And maybe even even other things, like plants, fungi, single celled organisms, or atoms.

I don't think that's a stretch. Presumably, the witnessed experience of smaller things, like atoms, are less vibrant and varied than bigger things, like humans. But it's hard to know.

But all of those things I mentioned above share a common lineage. All life forms on Earth are related, and are all built out of atoms. And so it's easier to draw an analogy between them.

But so the interesting question then becomes, I think, What other types of systems, besides Earth's life forms, might have sentience? And further, how would we know what stimuli might make such beings feel good or bad?

For instance, does a car have sentience? And does it feel pain if it gets into a car crash, or does it feel hungry when it's low on gas? For the record, I'm quite sure that the answer is no. But until we can very clearly articulate why cars aren't sentient, and have solid reasons that we can consistently apply to more scenarios, like an AI, I think most of the speculation we have about sentient beings is totally off the mark.