You were talking about instrumental values? I thought you were talking about terminal values.

How does a purely rational mind feel about the inevitable over-population issue that will occur if more and more lives are saved and/or extended by technology?

Overpopulation isn't caused by technology. It's caused by having too many kids, and not using resources well enough. Technology has drastically increased our efficiency with resources, allowing us to easily grow enough to feed everyone.

Does a purely rational mind value life less or more?

The utility function is not up for grabs. Specifying that a mind is rational does not specify how much it values life.

I was answering based on the idea that these are altruistic people. I really don't know what would happen in a society full of rational egoists.

In other words, pure rationality is cold and mathematical and would consider compassion a weakness. While this may be true...

It isn't.

A purely rational person would be nigh omniscient. If a combustible engine does more good than bad (which it does), a purely ration person would realize this.

If you want to know how we'd act if we just weren't biased about risks, but were just as imprecise, consider: would it be worth while to have been substantially more cautious? Barring nuclear weapons, I doubt it. The lives lost due to technological advancements have been dwarfed by the lives saved. A well-calibrated agent would realize this, and proceed with a lesser level of caution.

There are areas where we're far too cautious, such as medicine. Drugs aren't released until the probability of killing someone is vastly below the probability of saving someone. Human testing is avoided until it's reasonably safe, rather than risking a few lives to get a potentially life-saving drug out years earlier.

It doesn't behave just like water. It behaves like a simpler model of water. If you look more closely, the difference isn't what you'd expect between a good model of water and a bad model of water. It's what you'd expect between a good model of XeYZn and a bad model of XeYZn.

In other words, it would act like water to a first approximation, but instead of adding the terms you'd expect to make it more accurate, you add the terms you'd use to make an approximation of XeYZn more accurate.

In order for everything to work exactly the same, there essentially would have to be water, since physics would have to figure out what water could do. That being said, it could just be similar. If it models how XeYZn should behave approximately, subtracts that from how XeYZn actually behaves, and adds how H2O should behave approximately, and has some force to hold the XeYZn together, you'd have to model XeYZn to predict the future.

Come to think of it, it would probably be more accurate to say that water is made of physics at this point, since it's really more about how physics are acting crazy at that point than it is about the arrangement of protons, neutrons, and electrons. In any case, it's not H2O.

Namely, I have no problem with discarding time and keeping causality.

Why would you? If you have time, the universe has laws and a boundary condition at t = 0. Causality arises from this. If you don't have time, the universe has laws and a boundary condition at configuration = big bang. Why wouldn't causality arise from this?

Your second paragraph is pretty accurate. Your third one, I don't really understand.

all possibilities, even those with extremely finite probability, will exist.

Possibilities with probabilities that decrease over time can be finite. Thanks to the expansion of the universe, the probability of a mind exist will likely approach zero, causing this to happen.

so now you've got a really bizarre set of laws.

Yes. I thought I was pretty clear on that. Breaking conservation of energy is barely touching how bizarre it is. It still acts like our universe, except where H20 and XeYZn are concerned.

What happens when you run electricity through water?

If you run electricity through XeYZn, it results in hydrogen and oxygen. If you even have H20, it will immediately turn into zenon, yttrium, and zinc.

It's not a small exception. The vast majority of the laws of physics would be specifying what XeYZn is, just so that it can make it act like water should. In accordance to occam's razor, this is astronomically unlikely. It's still possible. To anyone looking at it on a larger scale, it would seem just like ours.

Although, each wouldn't know about the other... so maybe they would be justified in inferring that the other doesn't exist.

Say what you will about them being justified. They're still wrong.