I made the following post in another thread. It was an interjection, and probably seen as a digression. I thought i’d post it as a new thread, in case it invites any discussion. I was thinking about such a post, to try to discuss related claims like “everything we can think of must exist”. Or something like that. My claim is that only that which is empirical exists. This prompts questions like “Then how can an idea change the world?”. Or something like that. [O_H - perhaps you want to excise the post from that other thread - “Why is materialism so popular?”]
The nexus between the physical and the nonphysical is abstraction, or meaning. So, numbers are not physical, but the meaning of a number is a preserved (durable) relation in our brains - and that relation is born of an abstraction. Abstraction is what we do to form words, or to recognise them. Numbers are like words in this way. The formation of what Russell calls words in the object-language are first-order abstractions, and further abstractions - to the point of nonsense or pure fiction are possible - so abstraction creates meaning, but can also destroy it. Abstraction has no conscience about such things, and neither do some philosophers.
This is actually one of the few subjects of philosophy not easily summarized in a brief way - it’s not complicated, but requires precision of terms and relations between them. It’s like a chemical formula - it looks more complex than it is.
What humans do is learn. We are able to preserve patterns and transpose them from one application to another. This is done in the brain (or the neurological system). When we read, we do not read each symbol individually. There are many musical compositions that are impossible to play if they are read note for note. Most, I would say. They are read in patterns. Patterns are a conceptual overlay that we can remember. I would have to look up the brain architecture to even begin to explain this physiologically, so i won’t.
Numbers are among the class of abstractions that are fictitious but not nonsensical. Their sense is found only in that they are part of a closed system, but that’s okay - because that is exactly why they are so useful. Numbers are only meanings. They must have been “discovered” only after a long series of hierarchical abstractions. And when we learn arithmetic as children, we mimic that process, to some extent. Some people never get past mimicking the early stages - we call them math dunces. I was an algebra dunce, until I studied Boolean algebra. Go figure.
In any event, the non-physical can “interact” with the physical in the human brain. Because we attach meaning to some nonphysical entities. That can lead to error as well as to something useful. But it’s not particularly mysterious. We learn. That’s just something that we do. And we can learn crap as well as gold. Philosophy, science, even religion are ways in which we strive to discern the difference.
To be clear about my scare quotes - it’s not the nonphysical entities that interact with the physical, it’s the meanings we assign them that do. And these meanings are learned information, stored in the brain. It’s a chemical rearrangement.