There is no "I" in "You"

[The Grand Nudger]

Can anyone answer why you're you.....and not me?

Sure.  Different life experiences, for starters.  

By what method of determination do I experience existence separate from your experience; 

We're attached to different sensory systems in separate locations.

more to the point, what determined both our particular experiences

Must be a great many things, assuming anything was determined at all.

and could one have been shunt in place of another (by what alternate means could I have experienced existence through your perspective or vice versa)?
age.

IDK, good questions.  Perhaps whatever we are -is- transferable...might not even need another human to pull the switcharoo with.  

Why here, why now...why not exist a thousand years from now or a thousand years past?

Because your mother and father didn't live a thousand years ago, and won't be alive in a thousand more?

Can this ever be coherently approached or determined materialistically/scientifically? Or, is metaphysics a fundamental necessity?

You;re asking about being, so ofc metaphysics will be necessary, but I doubt that you'll end up with a very accurate statements thereof without reference to the world we live in, or the method by which we generate sound propositions. More of an "and" than an "or".

[quip]

Can anyone answer why you're you.....and not me?

Sure.  Different life experiences, for starters.  

By what method of determination do I experience existence separate from your experience; 

We're attached to different sensory systems in separate locations.

more to the point, what determined both our particular experiences

Must be a great many things, assuming anything was determined at all.

and could one have been shunt in place of another (by what alternate means could I have experienced existence through your perspective or vice versa)?
age.

IDK, good questions.  Perhaps whatever we are -is- transferable...might not even need another human to pull the switcharoo with.  

Why here, why now...why not exist a thousand years from now or a thousand years past?

Because your mother and father didn't live a thousand years ago, and won't be alive in a thousand more?

Can this ever be coherently approached or determined materialistically/scientifically? Or, is metaphysics a fundamental necessity?

You;re asking about being, so ofc metaphysics will be necessary, but I doubt that you'll end up with a very accurate statements thereof without reference to the world we live in, or the method by which we generate sound propositions.  More of an "and" than an "or".

You're simply rephrasing the idea.

What determines "my parents";  "my sensory system"...etc in relation to yours prior to either of our extant circumstances?

[FatAndFaithless]

How about you tell us? Because I have a feeling you're building up to it anyway.

[quip]

How about you tell us?  Because I have a feeling you're building up to it anyway.

I don't know, let's see if we can find out more in the future.

[FatAndFaithless]

How about you tell us?  Because I have a feeling you're building up to it anyway.

I don't know, let's see if we can find out more in the future.

There, you answered your own question.

[drfuzzy]

Separate bodies.  Separate genetics.  Separate minds.  Pure chance.  Roll of the human genetic dice.  
No consciousness without the brain, no afterlife, no metaphysical woo involved.

By what environs and contingencies are these "roll of the dice" made....and who or what is rolling them?

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Yes.  The non-woo answer is "I don't know. Let's see if we can find out more in the future."

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That dodge begs the very question:  Can the future reveal it's secrets?

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Genetics.  Chance.  My Father met my Mother - would have never happened if he had not signed up for a summer apprenticeship in Nashville.  Pure chance.  Human biology.  No "who or what" or any outside influence needed, and none in evidence.

[quip]

I don't know, let's see if we can find out more in the future.

There, you answered your own question.

Well no, not really.  Just yours.

[FatAndFaithless]

Quip, if you're looking for reasonable potential factors, others have listed some in this thread already (chance, genetics, the obvious difference in personal experience, etc). If you're looking for a single, external, metaphysical answer well.. I dunno.

[quip]

By what environs and contingencies are these "roll of the dice" made....and who or what is rolling them?

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That dodge begs the very question:  Can the future reveal it's secrets?

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Genetics.  Chance.  My Father met my Mother - would have never happened if he had not signed up for a summer apprenticeship in Nashville.  Pure chance.  Human biology.  No "who or what" or any outside influence needed, and none in evidence.

Missing the point. 

What determined this particular gene pool as "yours"?

[The Grand Nudger]

What determines "my parents";

The owners of the sex organs from which you were produced, in most cases.

"my sensory system"...etc in relation to yours prior to either of our extant circumstances?

There is no me and you or us to compare or contrast "prior to our extant circumstances".  The question is nonsensical in context. Rephrase?