What is a mental substance? I think it can best be understood by contrasting mental substances with material substances. Think of material objects. They have properties such as their weight, whether they are dented, their colour and so on. These properties cannot exist by themselves, they are not freestanding, they belong to a material substance. Material substances, on the other hand, exist in and of themselves and are the bearer of such properties. Crucially, the properties can change but without changing the identity of the object or substance. So, for example, a table might acquire certain types of scratches, its colour might fade and so on as it grows older, but, despite these changes, it is still the very same table or the very same substance as it ages.
The concept of a substance and its properties also apply to the realm of the mental. There are experiences, for example, the experience of pain. But one can argue a pain doesn't exist all by itself, there has to be a self that undergoes the pain. Experiences, in other words, are seemingly always had by an experiencer, or self.
Our unaided vision can analogically be compared to the mental substance/self. The bespectacled vision can analogically be compared to one's particular mental state at any time, or our minds. Just as changes in the lenses influence our bespectacled vision but not our unaided vision, so too might changes in the brain influence our mental states (the properties of the self) but not our essence (the mental substance).
Notwithstanding the fact that throughout our lives our moods, cognitive abilities and so on continually change, we all still feel that we are nevertheless the very same self throughout our lives. This is so even for those who reject an afterlife. So the mental substance/self that I have described seems to align up to most peoples intuitions as to what they truly are.
Note that this self I have outlined is not the same as the sense of self just as a sense of a tree is not the same as the tree itself. Few of us would deny that we have at least a sense of self. But most professional philosophers reject this notion of an actual self, they believe it is an illusion.
Should there be an afterlife it is my view that it is this self or mental substance that will survive. In which case the self or mental substance can be referred to as one's soul.
Alleged problems with such a mental substance or soul
the idea that the brain does not cause the mind, but that it merely serves as a gateway for it, just like a radio set functions as a receiver and decoder of electromagnetic waves.
A radio set (or a TV, if you prefer) and the signal it receives are separate things, and so they can exist independently of each other ... Destroy the receptor, and you still have the signal. Obliterate the brain and you still have the soul.
However, Julien Musolino is not impressed with such analogies. He goes on to say:
a few moment's reflection reveal so many dead-ends, contradictions, and nonsensical implications that it will make your head spin.For starters, the receptor view of the brain doesn't even begin to respond to the challenge posed for dualism by what we called the fragility of the mind. If damaging only parts of the brain can annihilate just about every aspect of our mind, then by what miracle would the complete destruction of our brain following death leave us with all our mental faculties intact so that we can recognize Uncle Fred in heaven? If the soul needs a functioning brain to be able to think, see, and feel, then how could it perform these functions without a brain at all?
He also says:
Does the all-or-nothing radio-brain view entail that the soul signal gives rise to my entire mental life? Are the languages I speak, the memories I have, the skills I possess all the product of something beamed into my brain from above? My suspicion is that the reason I speak French and English is because I grew up in France and then moved to the United States. I am also convinced that my memories have to do with the people I've met and the places I've visited in this world. If certain aspects of my mind are the obvious consequence of my dealings with the denizens of the physical world, then what exactly is the soul signal supposed to do? Does it just make me conscious?
It seems to me that Julien Musolino doesn't appear to have any understanding of what he's attacking. And he essentially says nothing else regarding the analogy. So much for the aforementioned "dead-ends, contradictions, and nonsensical implications that it will make your head spin".
Two other people I've read that attack this analogy, but equally and independently misunderstand it, and in precisely the same manner, are Keith Augustine and Steven Novella. Keith Augustine is one of the editors and is by far the most prolific contributor to The Myth of an Afterlife (I wrote a ~13,000 word assessment of the arguments contained in that volume here). In that book Keith Augustine says:
It doesn’t take much reflection to see that a television receiver is a terrible analogy for making sense of known mind-brain correlations. For the analogues would have to be:
Broadcast station → Electromagnetic signal → TV receiver → TV program imagesExternal soul ↔ Interactive forces ↔ Brain ↔ Behavior
On this analogy, mental activity itself occurs in the external soul, just as the images of a television program originate from the broadcast station. But no damage to the local circuitry of your TV set can have any effect on the television program recording playing at the remote broadcast station, or on the signal that the station puts out.
Steven Novella raises the same objection, except he invokes the plot of TV programmes rather than the TV signal. He says:
A more accurate analogy would be this – can you alter the wiring of a TV in order to change the plot of a TV program? Can you change a sitcom into a drama? Can you change the dialogue of the characters? Can you stimulate one of the wires in the TV in order to make one of the on-screen characters twitch?
Well, that is what would be necessary in order for the analogy to hold.
But the fact that the TV set can have no effect on the TV signal/plot of a programme is the very point. Or, to use the simpler analogy, the fact that the eyeglasses have no effect on our unaided vision is the very point. For it is the TV signal/unaided vision that represents the mental substance/self. They are conflating the mind, which is what results when the self operates through the brain, with the mental substance/self/soul. In my many communications with Keith Augustine (e.g. in the comments under my amazon review of his book and elsewhere. Update 11/2/22 Amazon have, without warning, deleted all comments under customer reviews!). I have pointed out his misunderstanding on this issue, but in his responses he has always ignored this particular point. I'm also pretty sure I've pointed out this misunderstanding to Steven Novella too.
So long as we feel we are the same self as time passes the default assumption should surely be that this is correct unless compelling reasons are advanced to doubt this. And they would need to be compelling indeed since I'm sure the vast majority of us are convinced of our persisting nature, at least from our births until our deaths.
Finally, is such an enduring self/mental substance consistent with the notion that the brain somehow produces this self? Consider that my brain changes all the time. Could a constantly changing brain produce a self that is unchanging? Arguably this is problematic. Hence this notion that we are mental substances implies both an afterlife and indeed "beforelife". And this is before we consider any evidence for an afterlife.