Dualism is the idea that people consist of two parts, one material (the body) and one material (the spirit).
The point at which these two interact has been debated for centuries (Descartes thought it was in the pineal gland). In addition, which functions belong to which side is also a topic of debate and speculation.
Now, I can see how one might think this. There appears to be a part of me that is not my body, for some indescribable reason. Like you lose a finger, and it's not like part of you goes with the finger, right? Not your identity. But, the thing is, lose part of your brain, and some part of your identity often does go with it. So this casts some doubt on the idea.
So I was musing on how this seems to basically be an excuse for ignorance. Does anybody really talk about the soul of a bacterium? No, because we can explain exactly what goes on inside a bacterium in physical terms. Biology has come far enough that most cellular processes (especially primitive ones) are well-understood chemical processes.
So that brings up the question: where do things start having souls? How about eukaryotes? They have nucleii, and complex organelles separated from the rest of the cell by membranes. This is a ridiculous jump in complexity, though it's on such a small scale we often don't realize it. Evolutionary biologists talk about the great "geneses" that constitute huge, unlikely leaps in complexity. The formation of the first DNA, the development of the cellular nucleus, the formation of multi-cellular organisms, etc.
Does life get a soul at any of these stages? Multi-cellular organism span everything from primitive algae colonies to incredibly-more complex mosses and worms, all the way up to us and organisms of similar complexity. It seems silly, though, to insist that these must have souls. Theses are parasites and pond scum on the lower levels.
Okay, how about complex plants? Conifers? Their more-advanced cousins, deciduous trees? Incredibly complex life-cycles. Usually each plant contains separate male and female reproductive capabilities. Plants evolve to interact with animal life in complex ways, like flowers and fruit.
But we understand everything that happens inside a tree. There is no reason to insist that there must be some immaterial component that is responsible for certain functions when we can see all of the tree's functions taking place in complex-yet-well-defined biological processes.
Okay, so we move into the animal kingdom, and we'll skip over the amoeba and worms, because we already rejected those. So let's move on to jellyfish. Still pretty simple... how about coral? Sea cucumbers? All those invertebrates that don't really have brains? No? They are still pretty easy to describe.
Okay, so give them a spine! Fish? Have you ever looked into a fish's eyes and thought "I can't see those without seeing a soul?" Fish are pretty dumb, even though they are a quantum leap above sea-cucumbers.
So maybe it's not the spine, it's the brain! The octopus, the cuttlefish, and the squid are intelligent creatures. They have been compared to certain "higher" mammals, in fact. So do these have spirits? Well, we certainly can no longer 100% account for their behavior. So it seems that we have crossed some barrier here with a complex brain.
So let's go to things with brains between the complexity of the octopus and the fish... maybe shrews, or cardinals? (..a dumb bird. Don't believe me? I have a story about cardinals.) How about ants, their behavior is extremely complex, but their brain is relatively simple. Frogs? Lizards? This class of creatures seems pretty ambiguous. They are not entirely predictable, but they are pretty well understood, and it doesn't seem reasonable to demand a soul for these.
But what is the difference between a frog and a dog? Do dogs have souls? They have large brains, and have emotions and personalities that seem similar to a human's. Ultimately, though, the difference between the frog and the dog seems to be one of scale. Does god decide to start shoving souls in when things are smart enough to pretend to have one?
I know I've rambled a little, but I just want to demonstrate that the difference between a human and a worm is simply one of scale, and fish are practically our cousins. Where do we start insisting that there is some incorporeal part that accounts for the behavior of an animal?
So let's pretend for a second that humans and only humans have souls. Never mind that almost all human behaviors are not unique in the animal kingdom (though there are a few little gems, like fearless use of fire). (And no, texting doesn't count. It's just communication and tool use that is more sophisticated than, but not fundamentally different from, that of apes and dolphins.)
Psychology is rapidly unlocking the secrets of the human brain. There are things that are interesting (like the guy who had a certain brain injury and couldn't tell his wife from a hat, or even my professor at BYU who has a disorder that makes it nearly impossible to recognize faces). There are things that are terrifying (like those scientists who figured out how to influence you decisions using magnetic fields!
So what behavior remain, that our brain is not responsible for? In the spirit world, are we utterly incapable of recognizing one another? Does the magnetic field mess with our spirits? (that violates the immaterial aspect, which, incidentally, is already violated by the idea that the spirit influences the body)
The whole thing gets a little hokey when you try to reason with it.
Couple that like some questions like "if god can create souls, they can be created. Can they be destroyed?"
Kinda defeats the purpose, if our souls are not invulnerable. It gets worse with each question.
When we are little kids, we actually ask our parents about things like this! Remember? The questions that really seemed pertinent to us, but prompted nothing more than knowing smiles? "Oh, maybe we'll know that someday..." Why is it not okay anymore? Because it is taboo for religious adults to openly admit to thinking logically about their belief.
And now, I rip off j-dog.