I have seen claims on this forum that human beings have souls, and that souls are separate from bodies and can live on after the death of the body, either by being reincarnated or by going to a different plane of existence, a heaven or a hell perhaps.
It's a common belief. It's probably derived from some popular version of Platonism that was circulating in late antiquity. I think that the more Biblical idea is that people die and return to being inanimate matter ('dust'). Then at the end of time they are bodily resurrected by having their bodies recreated. Presumably their personalities return as well, but that picture is probably consistent (at least in that respect) with the idea that personality is neurobiological.
Some people equate the "mind" with a soul.
That's (kinda) how the ancients came to think of it. In ancient times the soul was the animating principle, whatever it was that made living things alive. Earlier and more crude versions equated it with the breath, we need to keep breathing to live and when we stop breathing, we die. So we read about God "breathing life" into things. The idea of ghosts as spectral vapors derives from this ancient belief, from the idea that the last breath of the deceased individual might still linger around.
By the time of Classical Greece, more thoughtful individuals were aware that a number of things went into being fully alive. So they tended to divide the soul up into multiple parts, based on their functions, and they classified living things by what kind of soul they were thought to possess, and by how developed that soul was thought to be.
There was a vegetative soul that even plants possessed. We might equate that one with cellular metabolism in our terms. Then there was an animal soul that
animated animals, that enabled them to
move under their own power. That's probably why Thales thought that magnets have souls, since they can induce metallic particles to
move.
And there are more psychological functions of a soul. There's
sentience, awareness of one's surroundings. There are passions and emotions. And, so the Greeks thought, there was the supreme function of the soul riding up on top:
reason. Only humans possess reason, hence we are the rational animals, our defining characteristic. And since the universe and the principles animating it appear to be rational, humans arguably share the faculty of reason with whatever the highest principles of reality are.
They (especially the Neoplatonists) generally tended to spin that idea in a religious direction, but it's an idea that physics today seems to share, simply as an a-priori posit, a
default.
On the other hand, the best available science suggests that the "mind" is purely a product of the brain, and there appears to be no good evidence for the existence of any "soul" that survives the death of the brain.
I'm inclined to agree with that, but I need to point out that it's controversial in the philosophy of mind. There's Colin McGinn and "mysterianism", there's David Chalmers and his "hard problem", Thomas Nagel and "What is it like to be a bat?", Frank Jackson and his 'Mary black and white' thought experiment and so on. I'm much more inclined to side with Daniel Dennett (and presumably you) on that score, but there it is. CC is very into this stuff.
https://en.wikipedia.org/wiki/Hard_problem_of_consciousness
https://en.wikipedia.org/wiki/New_mysterianism
If you believe in souls, here's your opportunity to explain why you believe in them, and to present your best evidence for their existence.
I think that I would agree with these rather anti-physicalist (though Chalmers insists he isn't that) philosophers of mind that there are real problems that need addressing here, in defining what the word
consciousness means and what it refers to, in understanding what 'qualia' are (assuming that they
are in any ontological sense in the first place, which opens up a new metaphysical can of worms), and in explaining how these things can arise from a wet squishy neural network.
Note: you don't get to have your soul by default. It is not enough to say "You can't disprove the existence of souls, therefore they must be real."
No, no, no, no. You can't place your thumb on the rhetorical scale like that. The only thing that you have control over (apart from banning those you disagree with) is determining what convinces
you. Whether other people's belief in souls (whatever 'souls' are thought to be, like 'consciousness' it's a word that needs better definition) is a default is up to them, not you. But beyond that, I'd say that the whole "hard problem" idea is kind of a spin on the default position, an idea that Chalmers and company hold to in part because they can't see any way to avoid it in physicalist terms.
We can't disprove the existence of unicorns either.
Well, you're
talking about unicorns and presumably your words have meaning. So unicorns exist in some sense, as a concept (or perhaps as a supposedly unactualized possibility). If we go on to say that 'Unicorns don't exist', we would seem to be asserting that there is no physical object of the correct biological sort (statues, literary accounts and drawings of unicorns obviously exist) that corresponds to the concept or the depiction. Which is a hypothesis that we can't really justify with evidence, since we have not examined what's on every exoplanet out in the wider universe. I agree that the likelihood of unicorns existing is probably so small that for practical purposes I assume that they don't, but it's not something that I (or you)
know. It's more along the lines of our own default.