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

That is absolutely untrue. My perception of the world might be profoundly different if I knew with certainty information regarding the existence of souls, or the independence of mind from body, or the source of the universe.

These are not trivial matters.

One could just as easily say that it makes no difference whether string theory is correct or not, as it makes no perceptible difference to one's life.

> 
> Is it still considered correct that a question with no perceptible difference one way or another [...] is inherently meaningless? 
> 
> 
> 

Could you provide a legitimate example of a question with *no perceptible difference one way or another*?

> 
> Positivism was accepted for a while in philosophy, but no longer. I am shocked by this.
> 
> 
> 

Why does this shock you? There have been many critiques of positivism, and virtually everybody (including many former positivists) believe that philosophy has moved forward since positivism's heyday. A.J. Ayer, in fact, famously said "I suppose the most important [defect]...was that nearly all of it was false."
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.