Question: There are many questions in philosophy of the following kind:

* Does my friend have a soul, or is my friend a zombie?
* Is the mind separate from the body, or is it the same?
* Where does the universe come from?

These questions have the property that, whatever the hypothetical answer would be, every one of your perceptions of the world is *exactly the same*.

It is a central tenet of Carnap and the positivists that one should consider such questions as meaningless, as an abuse of language. This position answers or moots most of the questions on this site. Positivism was accepted for a while in philosophy, but no longer. I am shocked by this.

Is it still considered correct that a question with no perceptible difference one way or another (like, "Are you a zombie?") is inherently meaningless? Why would anyone assign meaning to such a question?

Answer: 
It ***can*** be very meaningful, and just *how* meaningful will vary depending on the person and the question. **Even if it doesn't alter the observable (physical) world** ("your perceptions of the world"), **it is still meaningful if it alters *human behavior*.** In fact, it is strange to me that you don't seem to consider human behavior as a "perceptible difference" of the world.

Consider:

Does God exist?
---------------

* If God **does** exist, then the way things are in the world is exactly as he planned. He is here and has always been here.
* If God **does *not*** exist, then the world keeps on ticking, exactly as it has this whole time.

However, while the ***world/universe*** doesn't change, **our behavior towards the world can change**. For example, if I was provided solid evidence for the existence of God, I would radically change my life in order to conform to his will (if we are talking about the Christian God, perhaps I would start praying, repent for my sins, attend Church more, etc.).
I think that methaphysical theories (non testable in principle) are necessary and important to scientific research because they may be capable of becoming physical (testable in principle) theories.

Of course, if you understand clearly that your answers are completely equivalent experimentally, then you are saying just the same in different ways, you get into a purely formal ground, like when we state the equivalence of two different formulations of the same physical theory; but it may happen that the possible ways of showing the difference empirically are still not understood - think about the [Aharonov–Bohm effect](https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect#Potentials_vs._fields), e.g.