Something we use every day but think little about is money - but what is money?
A fundamental principle of money is that it is an exchange equivalent, it exists to enable or simplify trade. It is also an arbitrary unit, it can be anything, from jewels, to precious metals to pieces of paper.
Some people will call money a debt and the English pound notes carry a statement making them a debt obligation by the Bank of England, but that would place an inherent value on money.
Incidentally the view whether money is a trade equivalent or a debt leads to a slightly different treatment of it. Where it has value by itself (a debt) it become something to speculate with, where it is a means of trade it becomes just a tool in life.
Whichever opinion you chose, in the end either only has the value that the recipient ascribes to it.
This means if you offer me a 20 Pounds note, I could still value it at less than 5 Euros, even when the official conversion rate is somewhere around 20 Pounds = 22-24 Euros.
In fact, back when European countries had their own currencies, it was possible to pay with the Deutsche Mark instead of Dutch Gulden in some places - at a premium (to cover the cost of exchanging and a surcharge for the convenience).
The next aspect of money is inflation and deflation. Some people decry inflation, but the existence of inflation and deflation are necessary evils - and will exist with or without monetary policy.
The original logic was that inflation will prevent people from hoarding money and wealth for a long time and make use of it. Deflation on the contrary will make people save money as they expect to be able to buy items sooner, thus small inflation was considered desirable.
Inflation itself can come about in two ways, either by the government printing more money or buy an upward shift in both living costs and wages. Conversely, one can argue that a drop in the cost of goods is also a form of deflation assuming wages do not drop in parallel.
In the end, a currency cannot remain stable unless we create a table of prices for items maintained by the government and even then it would not be stable due to changes in wages, productivity, etc.
Some people will advocate a return to the gold standard as opposed to weighing currencies against economic performance, yet that will still offer the same issues and can put small countries with small gold reserves at a disadvantage with no other justification.
Some people will advocate currency experiments and local currencies exist in Europe, however these only have a very limited range. Trade has gone global and having a currency only valid within a few km defeats the purpose of having a currency in the first place. If we add in exchange mechanisms we again end up with the current financial system of central banks. Thus we cannot get around national currencies - not yet anyway.
One potential candidate would be bitcoin, but one must question how the system will develop in the long run. With the number of coins artificially limited, there will be a point when deflation sets it which could cause problems. (Japan has a deflationary economy for a number of years which meant that the price of Japanese products rose on the global market.)
We actually do not know what effects an inherently deflationary currency would have on global markets.
So where does this leave us:
Our monetary system has problems, mainly speculation in the form of investment banking.
But nobody has been able to offer an alternative system that would offer the same conveniences in life that would not end up being manipulated in the same way as current currencies are.
Whether we like it or not, we are stuck with our current monetary system and all we can do is try to limit the number of legalized "exploits" that enrich the few at the cost of the many.
We have a system that is inherently broken, but we have no better system to replace it with...
So, any ideas?