On the level of the matter, they are still identical. The same particles are swirling around, as before, and the dead twin has the same body that existed while he was alive, moments ago.

The dead twin has suffered some sort of catastrophe that ensures that he is not identical to the live twin. An autopsy will reveal a coronary thrombosis, it will not reveal that the dead twin's soul has departed the body.

So whatever distinguishes them, whatever separates living things from dead ones, can't be a material difference... even though we can observe its effects on a material level.

So a blood clot is blocking a coronary artery, preventing oxygenated blood from reaching the brain, halting essential chemical processes, causing the body to become inanimate. Dead. Centuries of study and logic have revealed the processes by which the human body operates. We don't know it all, but we have accumulated a lot of evidence. And yet, without any evidence, you dismiss all that as "effects".

That is the biologist definition of life, not the everyday parlance definition. If a rock was aware of its own existence, but did not have DNA or growth, we'd still call it alive because of it being aware of its own existence. Otherwise you would not call God a living being because he has no genetic code, growth, reaction stories stimuli, or self-replication.

That is not the definition of an inanimate object. Zygotes are living and grow, that's literally the opposite of an inanimate object.

They have no brainwaves, no brain activity, and consequently, no awareness that they exist. That's the definition of an inanimate object.

The world soul doesn't add anything to that description of life.

You don't have to use the word soul but I don't think you can just dismiss the idea as not adding anything. What biology classification of life doesn't consider the difference between different levels of animation? How about a mushroom, is it alive?

I mean that rationality is on a graduation

I would call that a hierarchy, which seems to be Aristotle's approach.

Rationality may require sensation and memory. A 1st trimester fetus has none of these.

How can we know that? Doesn't the presence of a brain, one that is different than other animals, at least signify that those characteristics are developing?

Calling being alive a soul doesn't delineate the concept of life anymore than just saying x is alive. Biologists have created a list of things that determine if something is alive or not. The world soul doesn't add anything to that description of life.

By gradual, I mean that rationality is on a graduation. My fault I wasn't clear. My dog is more rational than a newborn. Many animals exhibit a degree of rationality, whether it is how they communicate or their problem solving capabilities. Rationality may require sensation and memory. A 1st trimester fetus has none of these.

It sounds like you agree that rationality is a real quality but that you don't agree with the word "soul", since you reject Aristotle metaphysics do you have a good description and way to delineate the concept of "life"? It seems as if that is the main purpose of his description of "souls".

If you consider rationality to be a gradual process, what would you consider the first objective sign of it to be? I would choose the first neocortical cells as evidence of the beginning of the neocortex as an objective sign of the beginning of rationality. I agree that it is a gradual process, and I don't have to tell you but the Catholic teaching is that the process is never complete until we experience the beatific vision

I don't think it makes sense to talk about rational souls. Rationality is gradual. A fetus doesn't exhibit rationality

When do you think a rational soul is present?

I reject his metaphysics, but it doesn't bother me.