Two frameworks of economics

One is price theory. It says demand and supply. The demand goes up, the price goes up too, if the supply is limited. And the price goes down when supply goes up, if the demand is limited.

The key point is, you cannot observe the demand in the economy. We can add up the total number of a kind of stuff sold (bought). But that is not the demand. The demand is abstractive. It not means we can put the "demand" in your analysis carelessly. Actually we need to think a lot about the "demand" first to have a clear idea, before going to further analysis. Once we have a good understanding of the "demand" of a specific economic phenomenon, more than half of the explanation is done.

Another is monetary economics. Before we have fiat money, I mean when we were using the gold or silver or both as the intermediary of exchange, we have no series problem which needs to explained by monetary economics. We can simply put gold or silver or both into the analysis by price theory. Since the government can create money (almost) costless, many and more economic phenomena are out the range of the ability of price theory. Monetary economics fills the blank.

Monetary economics defines and re-defines a lot of words, "inflation" for example.  When we say inflation, we are doing analysis of monetary economics. Hundreds of analysis on newspapers or television or academic journals are full of "cost-drived inflation" or "demand-drived inflation". They are simply wrong by mixing these two frameworks together which are un-mixable. My viewpoint is, "inflation" is all and only monetary phenomena.

(You can find more by reading "The Impact of Milton Friedman on Modern Monetary Economics: Setting the Record Straight on Paul Krugman’s 'Who Was Milton Friedman?'" , if you are interested in monetary economics.)