Suppose people want to spend 30% of their income on unobtainium. But the supply is zero, because unobtainium is unobtainable. So there's an excess demand for unobtanium equal to 30% of GDP.
What does that imply, all you students of economics?
By Walras Law, the sum of the values of all excess demands must equal zero. If there's an excess demand for one good, there must be an equal and offsetting excess supply for at least one other good. So if there's an excess demand for unobtainium, it is impossible, even given perfect price flexibility, for there to be equilibrium in the markets for all the other, obtainable, goods. There must be an excess supply of obtainable goods, equal to 30% of GDP.
An excess supply equal to 30% of GDP would cause a really big recession. Prices for all the obtainable goods would fall without limit, but can never reach equilibrium. They can't reach equilibrium until the excess demand for unobtainium disappears. But that can never happen.
God, but I hope nobody ever wants to buy unobtainium. I hope nobody even thinks about unobtainium. Who knows how desirable they might imagine unobtainium to be, and how much of their income they might want to spend on unobtainium? They might want to spend 100% of their income on unobtainium, so the excess supply of all the obtainable goods would be 100% of GDP. Nobody would be able to sell anything, if we let people's imaginations get going. GDP would fall to zero.
Is it morally responsible for me to publish this post, given what I know about Walras' Law? Try very hard not to think about unobtainium! If you think about unobtainium, you might want to buy some, and that would cause a Great.....CRAAAAAAAASH! Too late!
Of course it is. I know that Walras' Law is total rubbish. If we want to buy more of something, but can't, because the sellers can't or won't sell more of it, we will have to spend our income on something else instead. We know we can't buy any unobtainium, so we spend the income we can't spend on unobtainium on obtainable things. There's an excess demand for unobtainium, but no excess supply for all the obtainable things. Walras' Law fails.
Money is the only dangerous form of unobtainium. If we want more money, but can't buy more money by selling other things, like our labour, we get more money by selling less money -- by buying less of other things. That's what causes an excess supply of all the obtainable things.
That's about as clear as I can get it.