Supply and demand as a determiner of price is proof of the labour theory of value.
If demand increases supply should increase, but sometimes supply cannot increase, why? Scarcity of materials required to produce the product. What are the materials? Labour and material things.

But all material things technically speaking can be made from one another, if nothing else we could initiate nuclear fusion or fission and create the required elements and then the required molecules and arrange them into the correct... but of course we can't do that, why? Because it would take a massive amount of work for us to do that, at least with current levels of productivity, it's not viable as a productive process.

So the only really limiting factor on supply is human labour (I suppose once we get to the point at which all matter in the universe is human flesh then finally we hit another limiting factor* - but this is not a factor in day to day economy). So the prices which the economy settles on, on account of the law of supply and demand, are a confirmation of the labour theory of value.

I've been reading an account of the planning system in the Soviet Union which had a great many failures, and the fact that their mathematicians tried to come up with notional "prices" on which to base their plans and filter them down that for the most part were not reflective of the real world at all. It seems to me that the answer to that is to go back to the basics - supply and demand.

Now supply is easier to measure than demand (especially since demand in this sense is demand as modified by price) - but with networked computers in the homes of almost everyone in the western world and in a significant proportion of communities if not individual homes elsewhere, it shouldn't actually be an insurmountable task to measure demand (even demand modified by "price").

You could say "it's superfluous, we already have a way to measure these things and it's the market" - but lets be honest, the market as a means of distribution (like I said in my last post - of both products AND work) has significant inherent failings. As a driver of production it has a tendency to run out of control due to accumulation for accumulation's sake, causing economic turmoil, environmental destruction, monopoly development, warmongering for profit etc etc.

*Thinking about it energy is also a limiting factor but we can convert matter into nuclear energy now so this statement still stands.